I think re-taking both an in-house test and road test would be in everyone's best interest, starting at 65.
The in-house testing wouldn't have to be a written one, although it might not be a bad idea; but it's more important to test eyes and field of vision/reflexes.
I figured I'd Google first, to see if there were any current laws, but apparently there aren't any. I did, however find a transcript that was pretty interesting:
transcript
Night vision, reflexes, ease of mobility, comprehension and ability to concentrate all factor into driving well. Of course, many younger drivers aren't optimum at all of those, but whatever level one is at at 30, it stands to reason that declines after 30 or 40 years naturally. Add in certain illnesses and the declination would be more; a friend of mine, only 60, has had 5 strokes. He can drive well and has to because of his line of work. He will no longer make left turns unless there is a traffic light and lane strictly for that.