A really clever mechanic, executed very well! This feels pretty polished, and it's fun to play.
The aging mechanic is essentially a timer, but I think it's an interesting concept that could be pushed a bit further. E.g. maybe having it go a bit slower, and then having the older character be able to do different things than the younger one, allowing you to solve different parts of the level at different times?
Also, with a game this fast-paced, it feels like the controls/platforms could be a bit more generous (e.g. some coyote time). One of the levels had me bump my head into the ceiling or not jump far enough over and over.
Overall, a nice game!