Whoa there, cowboy! I recommend starting out on a Squier just in case you don't like it. And just because you're 12 DOES NOT mean you need a short scale bass.
Good call on starting with a Squier, but the short scale isn't really that short. I have a Mustang (guitar) and it works very well for me.
El Gearo:

Fender '69 Reissue Mustang
Fender American Standard P-Bass

Fender Blues Junior
This here looks like a job for the forums resident Mustang owner...that'd be me!!

The vintage white one looks better in my opinion, hence I bought that one, it's pretty well constructed bass, at first the tuners wern't great, but they've been behaving themseleves of late, so I cant complain.

This aint no modern sounding bass however, it's distinctly vintage sounding, with only a couple of tones, it plays well however, and its a fast neck!

BUT, it is a short-scale, meaning it doesn't sustain amazingly, although the sthring-thru design helps combat that, and it has amazing clairty considering, the main problem is it only has 19 frets, at first I figured that'd be ok,. but I do miss the 20th fret at times.

It is CIJ, meaning you're guaranteed a quality instrument at a very good price, I mean this is what sold it to me, along with the neck, its price to quality ratio, and the fact it's something a little different so you get noticed is cool.

any specific questions then PM me, or check out my review that's posted on this website, just search Mustang bass in the reviews section.
Quote by the humanity
I'm just joking Moog. you know nothing can tear our friendship apart, not even the fact we are miles apart, I am right there beside you, yelling, "Chug it, ya little wimp!"