This I have found to be hard. If you could help me I would really appreciate it.

In general the more lines on a face the older it appears.

In other words: WRINKLES. Give them jowls, give them eye bags. This is more subtle but, ears and noses never stop growing so make them bigger than your younder characters.

Adding on to what Phillby said, also remember that the skin tends to sag the older you get. Thats how the wrinkles are made. The skin sags most in the cheeks and forehead, and more lines around the nose will help define its size. Right underneath the eyelid is generally smooth, a factor of pulling skin. Also, their hands and legs tend to show their veins a bit more, if you're thinking more of full body stuff. More wrinkles in the hands, to show a long life of possible work. Add a scar or two. Everyone gets cut.

- more lines on the face
- smaller eyes
- rounder faces as opposed to "V" shaped faces
- more realism. For some reason, old people in anime don't look "out of place" even if they're drawn realistically.