I'm not at my computer or I'd demonstrate how I achieved this. I'll edit my post later. The goal to achieving an older look is exaggerating older features. Naturally, these models exaggerate young features with an anime style. Big eyes, small mouth, big head, these are all associated with younger looking models.
The first thing you wanna do is find a face model of an old man. Study the facial structure. A strong jaw line, stiff nose, and tough or wrinkled skin is a good start. Next you can play with the eyelids to make it more 'droopy'. Then size the head down in relation to the body. Smaller eyes, moles or wrinkled. Glasses could help if you can't get the eyes to look how you want.
Context is important too. When you introduce them into the game as an older figure, the player will likely use some of their imagination to aid in your effort to make your model look older.