Calzaghe is a tough case. Physically he is past his prime, slowing slightly, not able to sit on his punches at all anymore. But mentally he is at his absolute peak, he has a freakish boxing brain which pulls him through and leaves opponents rendered thick by comparison. Hopkins is pretty similar, could say DLH, Mosley and a few others are the same. That is what makes them GREAT rather than just VERY GOOD.
This may be the case, but the reason its so hard to pin down Calzaghe's prime is his reliance on physical gifts rather than textbook skills, but also his adaptability and ring IQ which have come on greater in later years. I don't think they've suddenly come aged 36 though, they've been there for a while. But I think we can say with relative certainty at 36 years old he is quite a few years past his peak, the speed and reflexes are down for me.
Calzaghe's peak physically is earlier than his mental peak so the balance between the two should have been around 2004. However, Joe was all over the place at that time with factors outside of boxing causing problems.
I agree that at 36 he is not in his prime as he is all about energy and keeping busy, as you indicated in his reliance on physical gifts. altho im not too clear on what you mean by gifts, i assume stamina, speed etc. but i find it hard to clearly state a period of his career where he was in his prime due to the far improved competition he has faced recently. his adaptability payed a big part in the kessler fight which i believe came with experience and calmness, his ability to analyse the situation and change his fight to beat the oponnent. could he have done that 5, 7 or 10 yrs ago against the same caliber of fighter?
If he can beat Kessler who is regarded as a A grade boxer by some, and a still competitive 43 year old Hopkins, in just the last 9 months. He can't be a million miles away from his absolute best, otherwise he would have been ridiculously good in his prime, and people like Reid wouldn't have lived with him - instead of having a very good shout for a rematch. He may well of slowed a little, but his boxing brain has improved somewhat with age. I think Joe himself stated something similar not that long ago.
Reid wasnt living with him for the first 5 rounds till Joe broke his hand. Beating a world class boxer like Reid 1 handed, even narrowly, is a serious achievement.
Calzaghe is undoubtedly past his physical prime, he's a bit older of course. However, he has reached a mental and spiritual peak over the last few years. And physically he's still pretty quick with his hands and feet, so you can't say that he's been overtly and/or apparently slowing down at all. He might just be a hint slower than he was at 29 or 30. If Calzaghe didn't look good against Hopkins it was because of technical flaws, and overall technical weakness in his envelope that's always been there. Not because he's on the slide or any of that.