There's no set age. If you gain weight you'll probably need a larger size. But, generally, as you mature your cup size will get bigger.
ANSWER:
As the above answer mentioned, it varies. Some girls physically mature earlier than others. My breasts started growing when I was 6, and by the time I was 10 and in the 5th grade, I was wearing a 28DD bra. And I was quite thin. Don't be in a hurry to have big breasts. The teasing and stares can be very hurtful.
The biggest bra size that us girls can have so far is 38kkk
Well if your talking bra sizes then... A A34 bra size is what most girls 12 to 13 wear. A B36 bra size is the bra size right after the A cup.
you can get a bra measure from primark
Your only option is to guess the girl's bra size. If you are not buying her a bra, you don't need to know her bra size. Try to find out something that's important to her instead.
36A, but every one is different
triple z
I wish but no
No, but there are preteen bras that would be in the size of an AAA bra size. It would probably be in the size 12-14 bra.
You can't. All you can do is guess.
what ever size you get or type
There is NOTHING wrong with you, most girls of 12 do NOT wear a regular bra, maybe just a cammisole or training bra!
That would mostly depend on what bra size you were previously, but on average 550cc breast implants will result in an increase of 3 cup sizes.