You are wrong SP Mauler, Walcott got better from ages 33-38 from 1947-1952. Also Archie Moore is EASILY the top choice here. Fought at his best in his mid-late 30s in the early 1950s. Hopkins too. Wlad, definitely.
He didn't get better with age he got help and took it more serious rather then 50hrs week laboring and fighting on the side. Not starving helped too.
It's been discussed that Archie was never clearly faster and sharper on footage than for his rematch with Durelle. Reportedly, Yvon was distracted with the imposition of personal issues which denied him optimal focus and preparedness for their return, but if he's isolated out of the equation so that one can just concentrate on Moore's performance, the Ole Mongoose does look impressive in his final match of the 1950s. Archie had a dozen bouts from 1960 to 1963, and only a young GOAT was able to beat him decisively. We don't know how long he could have continued, but he was still clearly on a different level when trained down to 175. Moore-Pastrano II for the LHW Title could have been a highly promotable rematch if Archie had chosen to return in a bid for his old championship. [He certainly didn't look underweight at 174-1/2 for Rinaldi II in June 1961, his final match at LHW.]
I would not pick Foreman incidentaly. I think that he went back as much as many fighters do, but that he just had enough to absorb it.
Glencoffe Johnson became a very wily operator as he aged. Became like teak in the toughness department as well. Not sure if he became significantly better, but he was still giving top fighters hard battles despite diminishing reflexes, so something's got to have improved.