I started lift accessed riding when I was 12. The bike I started on was a 4 inch travel bike. I was able to ride all blues and most blacks at Mammoth. My next bike was a 2002 Big Hit Comp. 6.3 inches in the back 5 or 6 in the front. I got this around your age. That bike was able to handle the double blacks.
I guess my point is ride what you have until it limits you. This will allow you to figure what you actually want. A slack DH bike is probably awesome (I don't have one), but it limits where you will ride and the type of riding you do (probably only going to want to shuttle). An all mountain bike might be more what you are looking for, but you never know until you start riding.
Take your current bike to a bike park, see if it limits you, figure out what parts you enjoy the most, find a bike that will grow with you, and (the most important step) convince your parents that if they love you then you NEED the $1500 bike you found online.
Have fun