12!
Well... I've had some experience, and ill put it like this.
at 12 years old, kids want to be "Cool" and have girlfriends. So they can show off, and make out.
Dont.
As SKG Said, Until they are at least 20, girls are lifeless bodies, they dont know what they want. Well, i would say at least 16, depending on what kind of girl we're talking.
If they wear short skirts, listen to hip hop, wear earings the size of their knuckles, chew gum 24/7, wear fake spray tan, and date different boys every day, Then you my friend are dealing with a "Chav". Thats what we call em over here.
Until theyre 20, theyre thick as planks. theyll just toy with you, and if every other boy goes out with her for a day, i doubt it would be any different with you.
However, if you get "Lucky" and do manage to go out with her, the chances are, it will last less than 2 months, and this isnt just "Chav" girls, this is all girls. and then, when youre 16, your mature, and she is mature aswell, the one time when youre old enough to realise relationships are supposed to last longer than a month, shell say no, why? it would be akward, to date someone you have already dated before, at youre age, youre probabily thinking, "It doesnt sound so bad" well, when youll get older, simple things like this can feel very strange.
Option 1) (A personall favoroite) stay away from girls altogether, wait until you are in college or university, you will be sure to find a girl there, who will also like you back. So what? it seems like a long wait. its worth it, i can tell you that. no point messing around with girls until youre serious, and even if you believe you are serious about a relationship now at this age, you have no idea what its like until you are 16.
Option 2) (Similar to option 1) however... asking this one girl out now, wont make a difference to what happens to you in college or uni, if anything it will give you more experience in what girls really want. So ask her. if she says no, deal with it, i mean, its one girl, it wont last anyway, and real relationships dont begin at 12 years old, if she says yes. then heck, it may sound rude, But you got yourself a practise dummy. See what she wants, what she expects from you as a boyfriend. then, when the relationship is over. Save all the infomation you gained, and use it when youre a bit older, youll know how to treat girls then. either way, this relationship would not last, or be a Real relationship (dont tell her that though...)
option 3) dont think about it... just ignore her, OR make very good friuends with her, until you are old enough, until you think you are ready for a relatioship, then ask her out,
The advantages...
1) you have ALOT of time to get to know her as a friend,
2)if she says yes, chances are it will last a long time as you will both be mature enough to take it seriously.
A NOTE:
About your friend, i had the same problem when i was 16, i was SO Close to getting with a girl, and my mate asked her and she said yes in the blink of an eye. if i was as smart as iam now, i would have left it. but i didnt, i broke his nose, we started a fight, ended up him going to hospital, with a fractured wrist, a broken nose, and a bleeding eye.
I was stupid then, i could have been friends with him still, but i wasnt thinking, and i havent seen him since.
-Stiggy