Money would be the first factor in not getting your private in a multi. Secondly, it would probably be a lot harder and even a bit more dangerous I think. Could you imagine having an engine failure on a ground ref maneuver? All that aside, 747, take it easy. You're thirteen years old, baby steps dude...first think about highschool and passing history class, then, if you have time, start working on your private. And if your still motivated start working on your commercial and instrument maneuvers. That way, when you turn 18 and take all those checkrides at once and enroll yourself into a COLLEGE, and if that college has an aviation program, you can immediatly begin work on your CFI. Then you'll totally be ahead of the game. I know this can be done because I had a flight partner that was an incoming freshmen that was enrolled with me in our CFI class. He got all his ratings at the FBO of the airport he worked at before he came to college. Motivation is a good thing, just make sure you dont burn yourself out before you have a chance to make it to the airlines.