In the last episode and evidence throughout the show would support the idea that when you cross universes at least 2 things happen, you dont age and you're presence causes death. Young black Henry was trapped in the non cursed castle rock for 27 years and didnt age a day. He then returned to his own timeline/universe and aged normally. White Henry or the kid, grew up in his timeline to become a full grown man instead of dying as a baby, and if the times link up he should be 39 as well. When he crosses over into the other castle rock he ends up in the 90s and didnt continue to age for 27 years. But you dont refer to a 39 year old man as a kid. So my only logical guess is that in his own timeline he could be younger than what he was supposed to be in black Henry's time line. The only thing is evidence doesn't support that, he's still friends with molly who's still older looking and even his wife seems like an older woman. But idk maybe it's just a plot hole, we wont know until the last episode is up.