My father is almost 90 and fluctuates sleep time anywhere from 6 to 9 hours a night. Usually averages about 7 or 8. He is in good health but does have a pinched nerve in his arm which is what wakes him up early sometimes. Once he's awake, though, he doesn't stay in bed. If he's doing okay mentally and physically, I don't see any reason to worry about his sleep time.
Chrysler, my parents were in pretty good health for being in their 90's and they use to brag that they only needed 6 hours of sleep. Of course, they never included the long nap after breakfast.... the nap right after lunch... and again an long nap after supper :)