I do think mileage matters more than age. But it's not specifically fight mileage, but more general body mileage - including sparring brain damage and physical wear, like joint injuries and injuries that aren't rehabbed well (either due to rushing or due to subpar protocols). I also wonder if cutting weight does any long-term damage that affects recovery. Some fighters can continue their elite MMA career with more wear-and-tear than others, due to their style. Eg Mousasi was always flat-footed and didn't rely on quick movement. Some fighters have built the skills to easily transition to a different game as physical limitations come along, eg Usman's knees are bad but he'd already become well-rounded enough to follow other gameplans. Or Mousasi had a torn ACL vs Latifi, but had the skills to follow a jab-and-move gameplan to a decision, which for example Yair Rodriguez likely couldn't do, if he suddenly had knee issues.
I think age itself doesn't matter much tbh. Vascularity/blood supply to tendons does decline with age, causing slowed healing, but I don't think it declines that quickly within the MMA window of 20 to 40yo (declines quickly after childhood), especially with these athletes doing so much cardio. Same for testosterone levels. A lot of non-athlete "aging" is just wear-and-tear from lifestyle, like skin aging from sunlight, effects of stress from having kids/work or sometimes IMO mentality or habits, like "I'm fat cuz of my slowed metabolism", which is just cuz people stop exercising as they get older (often mid-20s nowadays. Less sports, more sitting, less walking) and believe they're too old for habits that promote staying physically young, either because of ideas about being physically older or because they think it's immature to do it (eg immature to dance, balance on things). Sure, it gets harder to maintain at later ages, but people give in more than necessary. It should be a sign to invest in the habits more, but they do the opposite.
Not sure about Max's chin being cracked. Yeh, a little worrying and not legendary status after being dropped vs Gaethje and rocked and then dropped/near-unconscious by Topuria, but we've seen plenty of guys get similarly finished and still able to take a shot well enough in later fights, even experienced guys who've been in many wars like Poirier, Gaethje and Mark Hunt. Plus Max took a few years off from hard sparring, presumably from when he was about 28-29. If he rests properly I think his brain will be ok for MMA. He said he does things for brain health too, so I wonder what that could be (maybe some cognitive exercises for TBI?).