Fake Adventure time movie poster

So we're all know that Marcy is a thousand years old vampire - former half human and মাশরুম war only happened when she was a little girl (less than 10 years old, i think) but there is another human whose name's Finn is just only 14...So why in the present ( which is after মাশরুম war 1k years), there is one human left in the land of Ooo(who is Finn) and he is just only 14? There are at least 2 explainations for that:

1.The humans didnt really "get killed" all in the মাশরুম war and some of them didnt get mutated after the war, they still re-created pure human generations until about 12 years...