7mm-08.........perfect for your age. I am 15 now but if I was your age thats what I would shoot. I shoot a 30-06, with precision accruacy, but I am also 155 lbs. and 6'0 feet tall (I dont really look skinny, I just dont weigh much). I would consider a girl that is 15 and a guy that is 13 to be very similar when it comes to shooting. I know a girl that hunts with a 7mm-08, and is about the same size as you descibed (shes 15). .270 may be better for long range, but 7mm-08 should do the job. + you probably shouldnt be shooting over 200 yards. I still haven't shot anything over 150 yards, but this year I am "sort of" changing my equipment to where I will be able to make that shot. I am using a high mag scope (Nikon Buckmaster 4-12x50) and I think I will be buying some Hornady Ligh Magnum Loads for my 30-06, just in case I need a lil more getty' up.

If you have already decided on the .270, then that is fine, but a 7mm-08 would be a better choice that can last longer (but still very close to a .270) I would not go with anything under a .270 because it won't last...you'll out grow the gun. Actaully for a 13 year old a .280 may be perfect, but it is just harder to fins bullets for that rather than a normal caliber.

After that, it all depends on the amount of money for a scope. If you have a little over a $100 then I would go with a swift. (Perfect for a .270 or 7mm-08). But iff you have a little more go with a Nikon Prostaff 3-9x40 (only model) that will cost you about $150. Next would be a Leupold rifleman, about $170, then a Nikon Buckmaster, about $200+. Once you get higher than that you can look into the Burris' and the better Nikon's. You could also get a vari X I for about $200. It's all up to the amount of money you have, or your parents are willing to spend. Maybe you will be lucky and be in a family that hunts, because I just spent $400+ OF MY OWN MONEY to buy a new set of optics (binocular $60, scope $270, and rangefinder $99) pkus a few other things.

Good luck with your decision and I hope this helps you make a choice.