I enjoyed the game. Mainly writing in response to Maggie who didn't quite get it and others who are having trouble understanding the ages.
1. Baby is small, relatively quick, and causes trees to be at their shortest (remember you are not just aging your are controlling time). Baby will float through water (at least I think it is water).
2. Adult is also quick and can jump. Trees are at their second shortest. Adult will float through 'water'.
3. Old Man, is slow, cant jump but can use his cane as a weapon. Trees are at their second highest. Old man will float through 'water'.
4. Dead is immune to fire, but can be crushed by moving objects. Trees are at their tallest. Dead will sink through 'water'.
I must say I wished I had read the instructions prior to playing because I found cycling with the up/down w/s to be annoying as I would pass my desired level. I did however win the game in short order doing this so I certainly can't complain. I did very much enjoy the mechanic and I thought you used it well. I also think you could take similar mechanics to the trees and apply them to other elements in the world if you were so inclined to expand the game. I was kind of disappointed that the game ended when it did.
Overall I think this was well worth a play.