Actually, it is impossible to guess the age of the clown by its size. Even if you bought a w/c "baby" clown, it could be several years old by being the submissive undeveloped juvenile rank of a family of clowns (female, male, then several undeveloped juveniles is common...the juveniles will only sexually mature when the split off to form their own pair, or when the dominant juvenile develops into the dominant male after the loss of the dominant male or the dominant female (in this case, the male would become female and the juv would take his place...they maintain their hierarchy.)
So the only true way to know the age of a clown is by getting one captive raised. A juv could be a juv for years in the wild....or be lucky enough to jump right into the sexual positions. You can't tell by looking at them.