Boxers who were better in their 30's than they were in their 20's?

you really mean early 30s, as that includes a prime.

for instance, Froch, Calzaghe, 3G and Stuart Hall rose to world title level in the early to mid 30s.

 

Hernan Marquez barely made it to 30. He hasn't fought since.
Dude was legitimately done by the time he was 25. But he hung on for nearly 5 more years.
Diego Corrales was washed up before he hit 30.

 

Micky Ward, Wladimir and Vitali Klitchko, Davey Hilton, Joe Calzaghe

 

Plenty of fighters are in their prime at 30. It's actually kind of normal. Over 35 though is rare and pretty much close to impossible.

 

Martinez yes I agree.

Hopkins mabye, but he was highly regarded even back in the early 1990s. The only guy who didn't just get wiped out by prime Roy Jones. The Hopkins that Jones fought was 28. And that was a way better version of Hopkins that Taylor fought and Hopkins was 39.

Marquez was at his best in his early 30s for sure.

Froch I don't know maybe. His level of competition was higher in his 30s. But I've seen some his club fights on youtube. He looks like the exact same fighter in his early career. Maybe even a tad faster and quicker back then.

 

I remember when anything over 30 was considered old specially below MW. Now the new bar is over 40.

 

Larry Holmes

 

All the above.

 

I think Froch discovered the style that worked for him the older he got. the kinda blitz attack he used to beat Bute was an example of what I'm talking about, he would throw 4,5,6 punch combinations and only take a half step back and then throw another combo, rather than back off after an attack. I'm not an expert & I'm sure some people will not agree, but it's just how I saw him.
Awesome fighter to watch, maybe my favorite British fighter

 

Hopkins by far, still a phenomenon