I haven't read Return of the Condor Heroes in a while, but here's what I remember:
It's stated that in no specific terms in the beginning of the story, when he fights Ouyang Feng on the rooftop, that he has improved drastically since the end of Legend of the Condor Heroes, which was about 10 years prior, making him about 30 years old. This duel provides a nice little progress report as it's practically a stalemate, and both of them are wounded in the end. Reading this, it would seem that whatever progress Ouyang Feng makes in the ~10 year gap, Guo Jing has caught up to him.
During the scuffle on Zhong Nan Mountain a short while later, he battles the Taoists using the Big Dipper Formation and manages to break all but two of their swords with his internal energy. However, he silently laments to himself that not being able to break all of the swords with one stance is a nagging sign that he has yet to reach his peak in power, and still has a ways to go until he reaches his maximum potential.
In the years following, when he's about 40(?) years old, we see pretty much the rest of Guo Jing in actual action; battling Jin Lun Guo Shi, hordes of Mongolian soldiers, and also battling the Guo Shi, Xiao Xiang Zi, and Ni Mo Xing and holding his own until he is injured trying to protect Yang Guo. Towards the end of the novel, when Guo Jing is in his late 50's, we don't see him in battle but it's assumed that he's pretty much peaked in terms of internal energy and skill.
So at what age can Guo Jing be considered as legendary as his moniker of The Northern Hero (Bei Xia)? Is it right from the beginning when he's battling a Great, or is it at the end when he is named one of them?