Being young isn't always an advantage in sports.
Imagine an 18 year old wrestling a 12 year old. Get the picture?
I believe that it would be in gymnastics - particularly women's. Maybe ice skating. Equestrian events, perhaps. Events where size/body mass/body shape play a real role.
Can't think of too many other "money" sports where it might matter. Certainly not America's big four (NHL, NFL NBA, and Baseball)