I know "age 30" is a loose guideline; there's no significant difference between the state of my body at 29 years, 364 days and at 30 years. Unless there is vagmagic someone hasn't told me about.
Aaannnd as with everything from the waist down, mileage varies greatly from user to user, but I was wondering if there's anything that could give me a rough idea, other than my current plan of "keep using size A until I stain the hell out of a few pairs of underwear". (I've got a couple years left til I hit 30, and size A is still working just fine.)
(Also, can I just say that even having lost my first cup 8 years ago to an act of dog, and having to buy a third cup last year after mistakenly thinking I'd lost my second, I love that I've still saved hundreds of moneys over traditional products. yay)