People under 13 are allowed to create a Nintendo Network ID, but only with the consent of a parent/guardian.
It is quite hard for Nintendo to find out your age, but if they do you'll be in trouble, I will warn you.
I don't want to influence you in any way here, but it's your choice what you do. Of course, you are 12, so you'll be turning 13 in less than a year. So you can wait until then, you could get a parent/guardian's consent, or you can lie about your age. I'm not saying anything though.
You can read here about what you have to do to set up a Nintendo Network ID.
Hope I helped. :)
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