
Dr.Guru Pathik explains why it makes sense for an Avatar to be capable of bending all elements at a young age.
Aang: "I will never firebend again."
Guru Pathik: "That is not feasible. You're the Avatar and therefore you're a firebender."
Thanks, Guru Pathik.
It makes sense for an Avatar to know how to bend all elements, as they're born with all four.
They only discover it when they actively try to bend the other elements. With the separation of the different nations, benders from other nations rarely interacted with each other. As a result, the Avatar usually grew up learning only the same stances, moves, attacks as their peers, without ever questioning whether bending could work differently.