My father was one of those people who had many issues with his Mosquito. The problem (as YZ noted) is it's not a Sig, it just looks like one.
Onto getting a gun... So just to be super duper clear I'm not a lawyer... But here's how I understand handgun ownership under 21 under Federal law, State law may differ so check your local laws!
You can either purchase from a private party (option A), or inherit or be gifted the handgun by someone you know (option B).
So option A, you can cruise places like armslist looking for private sellers in your area selling Mosquitos and ask them if they're willing to sell to someone under 21. I mean... What else can I say? Either they will or they won't, and if they won't then you ain't getting a gun, nothing you can do about it.
Option B, you beg a family member to buy one and gift it to you. You can't give them money to do this for you, as that would likely qualify as a straw purchase.
Option C, which you may not like... Is simply waiting until you're 21.