Who are some boxers who had really good chins and it faded as they got older

I would say as a general rule yes, however there will always be guys like James Toney and Oliver McCall that never or rarely get stopped no matter the miles or damage to the car. All of the accumulation of damage and fighting on when your reflexes are going (don't see those shots you used to) factor in.

Exanples of Guys who stayed around too long and age/wear and tear got the best of them.....Ray Leonard survived Duran, Hearns, and Hagler but was eventually stopped by Hector Camacho.

 

Kell Brook, Crawford took him out so easily yet he was able to take good shots from GGG and Spence when he was younger.

 

Roy Jones would be a prime example.

 

How does cutting weight effect Chin

 

Michael Katsidis is one. I realized that extended pounding from Guerrero had altered his ability to absorb a hit when I saw him hurt by Albert Mensah's punches. Couple years later, he was getting knocked silly by a single shot from Tommy Coyle.

 

Wach

 

Bert Cooper.

His chin against Mercer and Holyfield was solid granite.

A few years later he was getting stopped by people who would never have even hurt him in his prime.

 

Sam Peter had a granite chin when he was younger but gradually lost punch resistance.

Mitch "blood" Green was another. It was hard to watch the punishment he survived from Tyson but at the end of his career, his chin wasn't the same.

 

Basically all fighters are an example of this. Wars and age make every fighter's chin to go worse with time.

What would be more curious is fighters whose chin got BETTER with time. And I'm not sure that there is any example of that, since chin cannot be trained (apart from strengthen neck's muscles, that is not a big deal actually).

 

What about Kovalev? His punch resistance was garbage vs Canelo.

 

Tex Cobb went from having one of the best chins in history while taking the best Ernie Shavers and Larry Holmes had to offer, to being starched in one round by clubfighter Dee Collier.
I don’t think that was age... I think it had more to do with his strategy of stumbling forward hoping his opponent got tired from hitting him.
Sometimes, a concrete chin just seems to crack overnight.

Same thing happened with Doug DeWitt...
I remember him being hit by Tommy Hearns right hands for 12 rounds with a grin on his face. I just couldn’t believe someone with a face first style could take Tommy’s punches for 12 rounds.
His very next fight, he was stopped in 3 rounds by clubfighter Jose Quinones. I could maybe understand that result as Quinones could punch but then he was knocked out by Sumbu Kalambay less than 2 years after the Hearns fight... Sumbu didn’t have 1/4 of Hearns power.

Speaking of Sumbu. Sometimes a KO loss just seems to be a fluke.
Sumbu was stopped just once in over 60 fights.
He was knocked cold in one round by Michael Nunn, yet was never stopped before or after that fight including fights with Iran Barkley and Mike McCallum.