Are we not wrong to think only of babies in this context? I used to teach in what is called a "special school" here in England. There are various types of special school, but the one I last worked in included a special care unit for teenagers, some as old as 19, whose mental/educational development meant that, although they had almost adult bodies, they were like babies in most other respects. Most could not speak, feed themselves, or even use the toilet without help. They couldn't even understand why they shouldn't cross the street or touch a hot stove. And of course, there is a whole spectrum of mental ability, ranging from those youngsters I've just mentioned through to "gifted" youngsters who have remarkable abilities at a young age. So where on that spectrum do we "stick a pin" and say "That's the level of mental ability necessary to become a Christian!"?
But when it comes down to it, our God is great - if He wants to grant salvation to a person who has not the mental ability to understand the need of a Saviour, He can and will do it. And I'm sure we'd all agree that intellectual ability will not get us to heaven. Paul was a man of great intellect, both before and after his conversion, yet when the Lord Jesus Christ met him on the Damascus road, he was breathing out threats against Jesus and His church.