Here are all the times "youth" appears in the KJB, lets see how old the persons spoken of are;
Gen 8:21 And the LORD smelled a sweet savour; and the LORD said in his heart, I will not again curse the ground any more for man's sake; for the imagination of man's heart is evil from his youth; H5271 neither will I again smite any more every thing living, as I have done.
I believe this is speaking of at least a teenager, as babies do not have evil imaginations.
Gen 46:34 That ye shall say, Thy servants' trade hath been about cattle from our youth H5271 even until now, both we, and also our fathers: that ye may dwell in the land of Goshen; for every shepherd is an abomination unto the Egyptians.
Old enough to keep cattle
Lev 22:13 But if the priest's daughter be a widow, or divorced, and have no child, and is returned unto her father's house, as in her youth, H5271 she shall eat of her father's meat: but there shall no stranger eat thereof.
Old enough to marry
Num 30:3 If a woman also vow a vow unto the LORD, and bind herself by a bond, being in her father's house in her youth; H5271
Old enough to make vows
Num 30:16 These are the statutes, which the LORD commanded Moses, between a man and his wife, between the father and his daughter, being yet in her youth H5271 in her father's house.
Most likely teenager or older
1Sa 12:2 And now, behold, the king walketh before you: and I am old and grayheaded; and, behold, my sons are with you: and I have walked before you from my childhood H5271 unto this day.
Teen or older
1Sa 17:33 And Saul said to David, Thou art not able to go against this Philistine to fight with him: for thou art but a youth, and he a man of war from his youth. H5271
Teen or older
2Sa 19:7 Now therefore arise, go forth, and speak comfortably unto thy servants: for I swear by the LORD, if thou go not forth, there will not tarry one with thee this night: and that will be worse unto thee than all the evil that befell thee from thy youth H5271 until now.
Teen or older
1Ki 18:12 And it shall come to pass, as soon as I am gone from thee, that the Spirit of the LORD shall carry thee whither I know not; and so when I come and tell Ahab, and he cannot find thee, he shall slay me: but I thy servant fear the LORD from my youth. H5271
Teen or older
Job 13:26 For thou writest bitter things against me, and makest me to possess the iniquities of my youth. H5271
Teen or older
Job 31:18 (For from my youth H5271 he was brought up with me, as with a father, and I have guided her from my mother's womb; )
Child
Psa 25:7 Remember not the sins of my youth, H5271 nor my transgressions: according to thy mercy remember thou me for thy goodness' sake, O LORD.
Teen or older
Psa 71:5 For thou art my hope, O Lord GOD: thou art my trust from my youth. H5271
Child or Teen
Psa 71:17 O God, thou hast taught me from my youth: H5271 and hitherto have I declared thy wondrous works.
Child or teen
Psa 103:5 Who satisfieth thy mouth with good things; so that thy youth H5271 is renewed like the eagle's.
Child or teen
Psa 127:4 As arrows are in the hand of a mighty man; so are children of the youth. H5271
Young man, married with children
Psa 129:1 [[A Song of degrees.]] Many a time have they afflicted me from my youth, H5271 may Israel now say:
Teen or older
Psa 129:2 Many a time have they afflicted me from my youth: H5271 yet they have not prevailed against me.
Teen or older
Psa 144:12 That our sons may be as plants grown up in their youth; H5271 that our daughters may be as corner stones, polished after the similitude of a palace:
Teen or older (grown up)
Pro 2:17 Which forsaketh the guide of her youth, H5271 and forgetteth the covenant of her God.
Teen or older
Pro 5:18 Let thy fountain be blessed: and rejoice with the wife of thy youth. H5271
Young man
Isa 47:12 Stand now with thine enchantments, and with the multitude of thy sorceries, wherein thou hast laboured from thy youth; H5271 if so be thou shalt be able to profit, if so be thou mayest prevail.
Old enough to work
Isa 47:15 Thus shall they be unto thee with whom thou hast laboured, even thy merchants, from thy youth: H5271 they shall wander every one to his quarter; none shall save thee.
Old enough to work
Isa 54:6 For the LORD hath called thee as a woman forsaken and grieved in spirit, and a wife of youth, H5271 when thou wast refused, saith thy God.
Young man
Jer 2:2 Go and cry in the ears of Jerusalem, saying, Thus saith the LORD; I remember thee, the kindness of thy youth, H5271 the love of thine espousals, when thou wentest after me in the wilderness, in a land that was not sown.
Teen or older
Jer 3:4 Wilt thou not from this time cry unto me, My father, thou art the guide of my youth? H5271
Child or teen
Jer 3:24 For shame hath devoured the labour of our fathers from our youth; H5271 their flocks and their herds, their sons and their daughters.
Child or teen
Jer 3:25 We lie down in our shame, and our confusion covereth us: for we have sinned against the LORD our God, we and our fathers, from our youth H5271 even unto this day, and have not obeyed the voice of the LORD our God.
Teen or older
Jer 22:21 I spake unto thee in thy prosperity; but thou saidst, I will not hear. This hath been thy manner from thy youth, H5271 that thou obeyedst not my voice.
Teen or older
Jer 31:19 Surely after that I was turned, I repented; and after that I was instructed, I smote upon my thigh: I was ashamed, yea, even confounded, because I did bear the reproach of my youth. H5271
Teen or older
Jer 32:30 For the children of Israel and the children of Judah have only done evil before me from their youth: H5271 for the children of Israel have only provoked me to anger with the work of their hands, saith the LORD.
Child or teen
Jer 48:11 Moab hath been at ease from his youth, H5271 and he hath settled on his lees, and hath not been emptied from vessel to vessel, neither hath he gone into captivity: therefore his taste remained in him, and his scent is not changed.
Young man
Lam 3:27 It is good for a man that he bear the yoke in his youth. H5271
Young man
Eze 4:14 Then said I, Ah Lord GOD! behold, my soul hath not been polluted: for from my youth H5271 up even till now have I not eaten of that which dieth of itself, or is torn in pieces; neither came there abominable flesh into my mouth.
Old enough to understand dietary laws
Eze 16:22 And in all thine abominations and thy whoredoms thou hast not remembered the days of thy youth, H5271 when thou wast naked and bare, and wast polluted in thy blood.
Baby
Eze 16:43 Because thou hast not remembered the days of thy youth, H5271 but hast fretted me in all these things; behold, therefore I also will recompense thy way upon thine head, saith the Lord GOD: and thou shalt not commit this lewdness above all thine abominations.
Child or teen
Eze 16:60 Nevertheless I will remember my covenant with thee in the days of thy youth, H5271 and I will establish unto thee an everlasting covenant.
Old enough to make a covenant.
Eze 23:3 And they committed whoredoms in Egypt; they committed whoredoms in their youth: H5271 there were their breasts pressed, and there they bruised the teats of their virginity.
Old enough to have sex
Eze 23:8 Neither left she her whoredoms brought from Egypt: for in her youth H5271 they lay with her, and they bruised the breasts of her virginity, and poured their whoredom upon her.
Old enough to commit whoredoms
Eze 23:19 Yet she multiplied her whoredoms, in calling to remembrance the days of her youth, H5271 wherein she had played the harlot in the land of Egypt.
Old enough to commit whoredoms
Eze 23:21 Thus thou calledst to remembrance the lewdness of thy youth, H5271 in bruising thy teats by the Egyptians for the paps of thy youth. H5271
Old enough to have breasts.
Hos 2:15 And I will give her her vineyards from thence, and the valley of Achor for a door of hope: and she shall sing there, as in the days of her youth, H5271 and as in the day when she came up out of the land of Egypt.
Teen or older
Joe 1:8 Lament like a virgin girded with sackcloth for the husband of her youth. H5271
Young woman
Zec 13:5 But he shall say, I am no prophet, I am an husbandman; for man taught me to keep cattle from my youth. H5271
Old enough to keep cattle
Mal 2:14 Yet ye say, Wherefore? Because the LORD hath been witness between thee and the wife of thy youth, H5271 against whom thou hast dealt treacherously: yet is she thy companion, and the wife of thy covenant.
Young man, married
Mal 2:15 And did not he make one? Yet had he the residue of the spirit. And wherefore one? That he might seek a godly seed. Therefore take heed to your spirit, and let none deal treacherously against the wife of his youth. H5271
Young man, married
These are all the times the word "youth" is used in the KJB. I believe all of these are speaking of a young person who is a teenager or older, with the one exception of Eze 16:22 which is speaking of a newborn baby covered in it's own blood. There might be a few occasions where this word is speaking of a child younger than a teenager, but even here it is more likely speaking of an older child.
But it is obvious this word is primarily speaking of young persons at least in their teenage years or older.