I've heard of elf or human or dwarf captains in goblin groups, but never of someones own captured baby or child returning. Yes the fort checks how old someone on the map is (and they can die of old age then) but when off map while children grow older, they don't grow into adults untill they enter your map. Since the captured dwarf is a child, children don't grow up and children can't be part of an ambush/siege group (I think), it's unlikely to happen.
Exactly.
Abducted dwarfs cannot come back. Since the world outside is frozen in time they never grow up and never become part of the goblin civilisation. You can see in Legends mode that dwarfs that were abducted during world gen become members of the goblin civilisation once they grow up and often they will become baby snatchers. You can get ambushed by those of course, but never by a dwarf who was abducted from your fort.
Apparently anything that ever leaves your map cannot come back. If you get human diplomats every year, it will be a different one every time. In Legends mode you can see that the human civ gets a new law-giver every year since they started visiting you.
Age is only checked if something enters your map, like your king dropping dead on arrival from old age.
You can even see the thoughts of your abducted children, they always remain babys and have the "is happy to be free" thought as if you let them out of the bag...