Avatar: The Last Airbender primarily featured the adventures of its adolescent protagonists, but by the time of The Legend of Korra they became parents and, eventually, grandparents to younger generations. Since they were such astounding Bending masters at incredibly early ages, it might stand to reason that they only grew stronger over time.
However, not all of them did. Age seems to work differently in the world of Avatar than it does in reality, and it may matter far less to how powerful a Bender is than many might believe.
With how physical the art of Bending is, it's understandable why proficiency in the elemental martial arts would decline as a person ages. That sure seems to be the case with the main characters from The Last Airbender by the time of Korra. Whereas Toph was arguably the most powerful Earthbender in the world by age 12, the decades that intervened saw her admit that she was past her prime to Korra herself. Though she was still capable of considerable feats of Earthbending and Metalbending when the situation required, her allies Zuko and Katara proved less so. Katara never involved herself in combat throughout the series, and Zuko's short bout with the Red Lotus saw him quickly discarded without putting up much resistance.
In spite of that, The Last Airbender established early on that age did not necessarily need to work against an elderly Bender's strength. King Bumi surprised the Gaang in his debut episode when he shed his robes to reveal a hulking mass of muscles, a brief preview of the astounding Bending strength he soon displayed in a duel with Aang. Iroh proved physically mighty himself even without bending, shattering boulders with a length of chain during his brief arrest by the Earth Kingdom. Later on, while imprisoned by his own nation, Iroh bulked up in the course of months to become so mightily muscled that he escaped his cell with raw strength alone. What did Iroh and Bumi have that Toph, Katara, and Zuko seemed to lack?
The truth is that age has very little to do with it. Those who reach 112 in the real world do not even retain full mobility from their youth, let alone leap around and make maneuvers as swiftly as Bumi did when the Gaang first met him. Iroh was superhumanly strong even before reconditioning his body in record time. It seems that the physicality of the characters in Avatar is less reliant on the limits people endure in the real world, and more dependent on their level of activity and training.
Toph, Katara and Zuko were all in varying stages of retirement by the time Legend of Korra caught up with the older characters. Toph's seclusion in the swamp left her few opponents to test her skills against, Katara stayed out of violent affairs altogether, and Zuko's work as a diplomat and emissary meant his fight with the Red Lotus was likely the first he had seen in years. Yet each of White Lotus members seen in The Last Airbender, a group that included Bumi and Iroh, were shown actively engaged in battle on various occasions even before they stormed Ba Sing Se to prove their legendary power.
Of course, there are other X factors mitigating the hindrances of age as well. Aang's own age was extended over 100 years by the Avatar State during his suspension in the iceberg where he was found at the start of the series. His predecessor, Kyoshi, was even hinted in her prequel novels to have learned mystical arts for attaining immortality from a mysterious man she met named Lao Ge. The technique is intimated to be the explanation behind how Kyoshi lived to be 230, over twice the span of any lifetime known in the real world.
So when it comes to Avatar, pinning down a Bender's prime may have nothing to do with their age at all, but with the time which their skills were most honed, practiced and active.
