Have you ever noticed how hard it is to tell how old someone is? Some of that has to do with our own age and perspective. As we age we sort of lock into a frame of reference of how old -or young--we are so our perspective gets slanted about how old other people are.
But there's another reason and that is the physical health of the individual. Healthy and active, an 80 year old can seem younger than a 60 year old. There's a retiree I see walking each morning who has the great posture and walking stride - I'm sure he's older than he looks but the way he holds his body makes him seem younger.