So iirc, in the scene where Renpa spoke to the Zhao messenger, the kid literally introduced himself to Renpa who claimed he remembered him,
But then at the end of their exchange, Renpa called the kid by the wrong name despite the fact that this kid had just stated his name to Renpa minutes before. The kid even clarified his name under his breath after Renpa got his name wrong.
You might be inclined to think that Renpa just didn’t care enough to remember a name that he just heard 30 seconds ago, but Mougou stated that Renpa has a photographic memory so Renpa forgetting the kid’s name at all seems like a massive stretch to me.
I think this is Hara showing that Renpa is going just a bit senile with age. I don’t think it’s anything that will necessarily effect him on the battlefield, but we may see more little things like this from Renpa in the future, where he forgets things that are inconsequential or are otherwise irrelevant, but on the battlefield we won’t see any decline.
In terms of his strength? Nah, I don’t think he’ll decline. I think the only time we’ll really see Renpa fight in detail again is when Shin slays him (not a historical spoiler), and against Shin, Renpa will have awakened the flame that he sought to reawaken against Ousen and Kanki, so there will pretty much be no decline from him. Or if there is a decline in strength, it will be used for a narrative purpose (Renpa’s era having ended long ago and now is the new era) rather than his strength declining because of age.
I don’t think people realize how common old-man strength is among older men who keep themselves in shape lol. I have seen videos of 50 year old men easily out-lifting guys in their 20s because those old men kept up their training and stayed active, which Renpa is definitely doing.