Lil Shooter,
I would recommend using a 20 gauge considering your intended uses. They come in a lighter, slimmer package so it will be easier for you to handle. If you want to take a look at O/U's, take a look at a Browning Citori in a 26 inch barrel or even a 28 inch. Now this isnt a youth gun, but it is light, but the stock may have to be shortened for your length of pull. A perfect semi-auto (although you are left handed) would be the youth beretta a391. Its a compact 20 gauge and I believe it has a 24 inch barrel. The semi-auto system will help even more to absorb recoil. A final pump option would be the new youth benelli nova. Its a great lil pump for around $325. Their are actually a bunch of options out their for ya. Good luck finding one, hope that helps.