i am 23 years old and got my 1st cup when i was 21. i never have had any children and don't plan to for quite a while! I ended up buying the larger sized mooncup uk for three reasons:
1. lunette (before the recent addition of their super small cup) and femmecup only offer (offered-lunette) ONE size. this size is a "one size fits most" and they are the same as any other brand large sized cup. Also, the original (large) lunette is recommended for women 25+ years old... even though it is the same diameter as any other cup. (so why are the other large cups recommended for women 30+ years old?!)
2. many women reported that the size isn't much of a difference (afterall, it's typically 2-3 millimeters) and that a larger cup typically took care of any leakage problems.
3. the vagina is very stretchy. so again, what's another 2-3 millimeters? also, with the large cup, i'll (hopefully) never have to buy another cup for as long as i'm menstruating!