Sleep needs vary across ages and are especially impacted by lifestyle and health. Researchers cannot pinpoint an exact amount of sleep need by people of different ages. However, sleep requirements vary from person to person even in the same age group.
There is a big difference between the amount of sleep one can get by on and the amount one needs to function optimally. For instance, if one is able to operate on six or seven hours of sleep doesn't mean one wouldn't feel a lot better and get more done if one spends an extra hour or two in bed.