Oda hasn't given me a real reason to think otherwise.
Roger and Garp are like 40? And both have complete control over their AdCoC, and have had AdCoC for atleast around a decade? It's not like they pick up dfs later on or something. Pretty confident they are in their primes, I think this is around the same age as Shanks currently is so it checks out.
It's actually even more obvious when you look at Whitebeard, Oda made sure he has AdCoC + Gura + His Supreme Blade by God Valley. If Oda wanted the readers to think he wasn't in his prime he would atleast design WB in a way where he doesn't give him his supreme bisento which is basically a status symbol.
IIRC he doesn't have it here, or atleast it's not designed like the final product.
By God Valley, WB has everything his final product had.
The last thing I'm about to do is act like these guys are not in their primes just because I didn't like how Oda drew their fights in a flashback or that it didn't meet my expectations. I need real logical reason as to why, the ages are right there, the haki is all there, their weapons are all there.