In the OT, I seem to recall 20 or so being used to some point as an age of acountability (In that time, for those people, in that place). I do sometimes wonder. One of my own many diagnoses was a "developmental disorder," which basically meant I was (somehow...) both precocious and behind other people, by a good bit. I messed up royally in life, got saved at 28, and now I seem to have been restored. My older, wiser, Pentecostal friend tells me that God sees to the heart of people and our situations, and He's chosen to bless me with things because I was out there, no clue how to be in society, etc. But then, I wonder...what of people who end up dying in their mess? Like I a guy I knew back in the day who died of a pain killer OD at 17, or a cousin of an acquaintance who recently died of a heroin OD at age 18. I"m not better than them now, I certainly wasn't before The Lord saved me, and yet...I was spared things that should have, the way things usually go, meant death and judgment.
OK. I think one's opinion on babies and kids also depends on if one is Calvinist or not. Calvinists don't have a problem with people going to hell. To them, its a miracle that anybody gets saved. Arminians seem to stress "making a decision for Christ," so I guess there maturity and other issues would play a big role in determining personal responsibility for accepting or rejecting Christ, personal sins, etc.
So...I don't know...but, there's that OT verse..."I will show mercy to whom I will show mercy, compassion to whom I will show compassion." God is sovereign, after all, and I think there's a huge mystery there, as well there should be.