Model the player's head / body twice - model him in full health, then duplicate and age him by contracting his skin around the bones, adjust the face to become more skeletal, make him slightly shorter etc...
Then during the game, morph the player mesh towards the second old-age mesh as and when your game requires the player to age.
Obviously that will only work with an identical mesh just with the verts moved around. If you want bits to change radically you'll need have child objects that can be changed - for example model hair as an extra in various states of growth, then parent it to the main mesh, then replace it each time the hair changes with the new mesh.
You could always vert-mod bits like the nails, if you knew exactly which verts they were. Write a viewer program that shows the model with vert numbers superimposed, so you can check which ones they are. This will mean hard-coded vert numbers which you'll have to update if you change the mesh, but never mind.
You'll need to adjust the texture to match. Perhaps alter the colours to make the texture gradually go paler, enhance areas such as the cheeks, or just swap between a number of aged textures.
Then during the game, morph the player mesh towards the second old-age mesh as and when your game requires the player to age.
Obviously that will only work with an identical mesh just with the verts moved around. If you want bits to change radically you'll need have child objects that can be changed - for example model hair as an extra in various states of growth, then parent it to the main mesh, then replace it each time the hair changes with the new mesh.
You could always vert-mod bits like the nails, if you knew exactly which verts they were. Write a viewer program that shows the model with vert numbers superimposed, so you can check which ones they are. This will mean hard-coded vert numbers which you'll have to update if you change the mesh, but never mind.
You'll need to adjust the texture to match. Perhaps alter the colours to make the texture gradually go paler, enhance areas such as the cheeks, or just swap between a number of aged textures.