That credibility speech you just gave, then this:
And, once they figure out they're buying it from some young punk,
And the pot turned to the kettle and shrieked at the top of it's lungs 'BLACK!'
Is that supposed to raise your credibility? I don't get it...
To the OP, I have to agree with what others have said here, though I'll do it more respectfully than some. If you had a few more years of experience (not knowledge, actual RL experience) and references, did this full-time, and were a bit older (sorry to say your age will hinder you a bit) then it's possible.
Probably 85% of all my clients have zero computer knowledge other than turning it on. Thus, they don't have the ability to go to a place like newegg and order what they want. Furthermore, I ask that they stay away from BB and CC and other big retailers - even with me upping the part price a bit and charging to install it, it's still cheaper and more nerve-easing for them than going to one of those and having someone who half-knows what they're doing mucking around in their system.
I'd wait a few years. if you have friends or family who want new systems and you can convince them that, after parts and labor, you can build them a better system for cheaper, AND give extended support if they need it, then build to your heart's content
I'd limit it to friends and family though, for the time-being, again, just because of the age. After you do a few of them, you'll be able to see how much profit you're making and decide in a few years if it's worth it for you. The real money in this business isn't building computers, it's maintaining systems and software.