When people usually say prime they mean athletic prime. We both know this. Was anyone on earth a few weekends ago saying "Golovkin isn't in his mental prime anymore"?. No. Everyone was saying he was past prime, meaning that, in an athletic sense, he's past it.
You're oversimplifying. There's a chain link effect with regards to prime. Yes, all boxers decline physically from their prime condition but some alleviate that decline with technical and tactical growth so that the overall package when older is better than it was when physically at their peak. Experience can too plug the gaps. There's often a difference between physical prime and boxing prime. In Joyce's case that difference is significant. You can't deny this, the evidence is staring you in the face. Golovkin has had a much longer career and many more fights both as an amateur and as a pro so his physical and boxing primes were better aligned. You citing GGG merely highlights the impact of factors other than age. You have to factor in 'miles on the clock' - not only in terns of the number of fights/training camps - but at what age they started boxing. Again in the case of Joyce, this increases the difference between his objective physical prime and his real world boxing prime. Lewis is older than Tyson and no one would dispute he was much closer to his prime than Mike when they fought; it was a mismatch when 15 years earlier it would not have been. Why didn't they decline at roughly the same rate? How was Lewis able to dominate a fight he may well have lost in the early 90s? Because he was much closer to his boxing prime, that's why. If you argue Tyson had abused his body over the years, accentuating the physical decline, that supports my point of view too...
Prime is one thing: Athleticism. It has always been this way. You're just telling me that no matter what I say, your point stands. So there's no point carrying on.
Jesus, a boxers prime is very different to an athlete's, this has got a little silly now. Nobody is claiming that Joyce is a prime Decathlete, he's coming into his prime as a boxer as he becomes more experienced and still has more than enough physically. Just look at his performances, that's what you go by, not what age he is.