Not sure when he reached his prime, but Kaido got stronger after his 40s (after the Oden fight, per Oden's own call at death's door: "build your strength while you can!")... he possibly got in his prime at his 50s. Urouge also has about the same age as Jinbe and definitely got stronger during the timeskip (went from getting beaten by a single pacifista to be able to beat a Yonko commander), so Jinbe can surely get stronger.
It seems that most characters' primes in OP happen between the late 40s and the late 50s (e.g.: the admirals, Roger, Whitebeard, et al.), with few notable exceptions, but still somewhat old by real-life standards (e.g.: Blackbeard, Shanks, Mihawk). Maybe the younger strawhats will be the ones to break this paradigm, but so far Jinbe still has room to grow by OP's standards.