@Dominions Son
A D-cup on a 12 year old of average height/weight is likely considerably smaller than you imagine.
I was going to mention that. But then height/weight/age tables seem to slightly disagree.
Seems the one I found in a quick Google Search says the "average" 12 YO girl would be 5ft even(152cm), and have a chest measurement of 30.75"(78cm). (An average 12YO boy is reported with the same height/chest measurement)
So going with 78cm as the "under bust" measurement, a D-cup would be around the 99 to 101cm mark, or about 39 inches. So I'd probably ballpark that as protruding about 4 to 4.5 inches from the chest. (size 36D)
The bra-sizing chart I have shows +21cm is D cup for all chest measurements, so that's a non-starter. But shifting to cup volume, going from a 36D to say a 40D, which would be more typical for an "average sized 16YO" female chest measurement(minus breasts), you go from 710cc(for 36D) to 1000cc(for 40D). 710cc of breast tissue could get you a 40B bra or a 44AA(98 to 102 cm band size). (edit to add: and yes, by implication, that means most of the men on this forum are likely to have more breast tissue than that 12YO)
According to my reference "child sizes chart" average male height @16 is 69.25"(176cm), with a chest of 37"(94cm). female is reported at the same height, but 36.25"(92cm) on their chest measurement.
All that being said, describing characters by cup size is bad writing. Fixating on 12YOs, even "mature seeming ones" is also borderline for other reasons, unless you happen to be a 12YO.