there's three basic body types, endomorph, mesomorph... cant remember the other one... but it basically means skinny, muscular or liable to put on weight. every person is a somewhere between 2 of these types.
however, years of running can somewhat alter this; similarly a 'skinny' type will put on weight after being sedentary for many years.
every person, regardless of size or age, has an upper limit. it doesnt really matter how high your limit is, just as long as you keep aiming to reach it.