User blog comment:Sargenttacos115/Why are Sting and Rouge so weak?/@comment-117.227.194.37-20131110013056/@comment-7418318-20131110074255

Yep. Their Dragon parents were already knocking at death' door when they trained the Sabertooth Twin Dragons, so maybe they weren't able to train them as well as Igneel and Metallicana.

Also we don't know exactly when Sting and Rogue killed their parents. With the two appearing to be younger than Natsu and Gajeel (were before they went to Tenroujima) and the possibility of them losing their teachers even earlier than the Dragon disappearance in X777, their training was most likely shorter and less intense.