This is getting kinda old, since every example seems to uphold my theory. Here are the 1989 inductees to the Rock Hall of Fame.
Dion: Like most of the best rock and rollers, Dion still occasionally releases a fine album that reminds us of his talent. And like most of the best rock and rollers, those albums are reminders of his prime, not part of his actual prime. In Dion's case, that prime was the years with the Belmonts and the first part of his solo career, up through "The Wanderer," made when he was 24. Everything else is "comeback."
Otis Redding: Probably gets my vote as the greatest of all the soul men. Sadly irrelevant for our purposes here, since he died when he was 26.
The Rolling Stones: I've talked about them elsewhere in these threads. As great as any rock and roll band in history up through Exile on Main Street, occasionally brilliant for another ten or so years after that, mostly unimportant for the last 20+ years. Keef and Mick were 29 when they made Exile.
The Temptations: They make the Top 100 artists of all time list at Acclaimed Music, where they also get six singles and three albums in the Top 2000 lists. The last of these come in 1972. None of the Tempts ever managed solo careers that surpassed their work with the group.
Stevie Wonder: Wonder is #12 on the all time artist list. He places eleven singles and six albums on the Top 2000 lists. The last of these came in 1980, when Wonder was 30 years old. He began in the sixties, he was one of THE crucial artists of the seventies, he was only 30 years old as the 1980s began ... and for the last 20+ years, he's done nothing to compare to the excellence that was his peak.
I think that brings us to 35 Hall of Fame rock and rollers so far, and not a single one has had a career that would suggest that rock and rollers continue to get better the older they get.