Both ST and TPF allow for multiple 'sets' or 'populations' of different leaves. The TPF Oak model here has 4 separate populations of leaves. So to vary the 'aging' process is simply a matter of assigning the desired leaf texture to any one or more of them. Likewise, culling of individual pops can vary the looks of the tree in a very natural way. ST has the same capabilities. All of the model elements can be controlled by various forces as Dune noted earlier in the thread, both TPF and ST can do this. Both TPF and ST can create a sort of parametric 'timeline' for the model to simulate 'Age' and 'Health' and 'Season'. This essentially is the gradual substitution of varying textures and element density to reflect the changing seasons. The ST version that does this is rather pricey at this point and that capability hopefully will filter down to the more affordable Studio version in a future update. On top of all that there is the 'Wind' function in each app that can be used to further model the object. And watching the trees blow in the configurable 'wind' in real time in the interface is awesome! The wind sequences can be exported via FBX, MDD and Alembic. MDD is large and cumbersome: but either FBX or Alembic would be really nice to see in TG!
