At your age, if you don't hold up your end of the deal, you legally get off scot free, so I would personally never do business with an under aged person and many others will also refuse on those same grounds; nothing personal, it's just nice to have some legal recourse if things go tits up, or at the very least, not have people accuse you of bullying a child if you complain about a bad business transaction with someone who is under aged.

I suggest you just keep doing your own thing for now, maybe do some trades. It'll get you used to the mechanics of a commission; someone wants you to draw X and you have to try and satisfy them within a reasonable timespan, and you get something for it in return. But there's generally less pressure and stress.

That way you'll also work in your reputation a bit and have a bigger customer base by the time you do start taking commissions for pay.