Im 15 same size as you and im playing wit 18 year olds, sumtimes its a challenge but really if you hit their legs they will come down, great thing about small guys is that they can get lower easier, i wud never tackle high, in league there are quite alot of 2 man tackling, but you should still get low and hit the legs thats the key otherwise unfourtunatly u may get pushed away by the bigger boys, i probably have the best tackling technique in the team due to the fact thats the only way ill make a tackle if i go low. Hope you dont mind me saying but ur a wee bit light, you may want to try bulking up a wee bit to get some more strength behind the tackles (if that is sumthign ur stugling with) sorry i couldnt be more helpful im a Union player, hope this helps :p
You can still play but if you don't get heavier you WILL need to get faster!