Over the years, lots of people have wondered whether Qui-Gon Jinn as of his appearance in The Phantom Menace was past his prime or not because The Phantom Menace Novelisation and a few other sources put a lot of emphasis on TPM Qui-Gon physically declining. It turns out that the official Star Wars website tells us about Qui-Gon's prime:
"Jedi Master Jinn possesses strong features and a piercing gaze. Powerfully built and speaking in a grave and determined voice, he has a very intimidating presence. He wears his long hair tied back so as not to obstruct his peripheral vision. he remains a powerful warrior and a very active Jedi Knight." -- StarWars.com: Qui-Gon Jinn (Archived)
The source says that as of TPM, Qui-Gon is "a little past his prime" but "remains a powerful warrior and a very active Jedi Knight" i.e. TPM Qui-Gon is a very formidable, but he is a post-prime version of Qui-Gon in combat prowess by a tiny margin. So the Qui-Gon that we see in TPM is only slightly inferior to his prime self.