Actually, it wouldn't be racist, it would be ageist:-).
Sean, something that you should realize, being the competent teen that you are, is that not all teens are like you. Since we make rules that fit the majority (or often the lowest common denominator, which is even more frustrating), those that are competent are often penalized. This applies to lots of things in life, btw!
The parents could really only sign a contract as you describe if they planned to be responsible for all of the things necessary to look after the animal. Odds are, they aren't.
"...but if you don't sell to him then he will just get it of some body else. " By this logic, why not sell cigarettes and alcohol to teens too? After all, they'll just get them somewhere else if one person doesn't.