I think the turtles themselves were only like...one biologically.
They only spent a year in the sewers (sans Raphael, still on the surface with Casey), and have been together ever since for the past two or three.
Leo for sure would be hitting his early 20's if they wanted to count the years they already had as humans. At the time of their human deaths, they would probably have been seventeen, sixteen, fifteen, and fourteen (goddamn, Yoshi and Shen didn't waste
any time
).
So Raphael and Michelangelo would be the only ones left that are actually teenagers.
So in the future they would be all be pushing 40. And it's the second-youngest who actually looks his age
. Maybe Leo is a spirit after all.