Avatar Aang was raised par monks, although the identity of his real parents was unknown. Being raised par monk Aang was taught about life and to respect it, the ways of the monks are ways of peace and meditation, Aang was already a master airbender before he found out he was the avatar, he learnt and mastered all four elements par the end of the summer. And all his masters were part of Team avatar(katara, toph, zuko). And he was just a kid. Monks are also spiritual people thats why it didn't take Aang along time to master going into the spirit world.
Well in Avatar Korra's case she mastered three elements(water, earth and fire) when she was about 4 years old. She was strong and powerful, even without her bending Avatar korra can still stand up to anyone. Like in book2:spirits where she defeated Unavaatu(unalaq and vaatu) she did it without her bending, even though it took her along time to master air and the ways of spirits. She is a water bender and not a monk so it is a normal thing for it to be difficult at first but now she mastered it all and we can say that Avatar korra is a great heroine. Avatar Aang is a great hero, both of them are great even though they are so different.