The 1990 inductees:
The Four Seasons
The Four Tops
Simon and Garfunkel
Do I really need to continue? Ballard and the Platters never topped their 50s work. Darin doesn't belong in the Hall of Fame, but in any event, he died in his 30s. Frankie Valli looks good on The Sopranos, but his best work was done with the Four Seasons, a group he left when he was 28.
That leaves the Kinks, the Who, and Simon and Garfunkel. The Kinks rank 43rd on the Acclaimed Music all-time artists list, with seven albums and six singles on the Top 2000 lists. The last of those came in 1971, when Ray Davies was 27 years old. They haven't made an album of note in at least 25 years.
The Who are #10 on the all time list. They placed nine singles and seven albums on the Top 2000 lists. The last was in 1975, when the band was all in their late-20s/early-30s. Three years later, Moonie was dead. The band's peak was clearly from "My Generation" through Who's Next. They haven't come close to matching that peak for 30 years.
That leaves Paul Simon, who's done better than most. Simon and Garfunkel, of course, broke up when the duo was in their late-20s. Simon's solo career had a nice early run (three ranked albums, three ranked singles through 1976), and then a true renaissance with Graceland, recorded when Simon was 45, and a nice followup a few years later, Rhythm of the Saints. With his Central Park concert just before he turned 50 years old, Simon looked like the rare rock and roller who maintained his peak throughout his later years.
Alas, his career has gone nowhere since then. He toured with Garfunkel, he worked for years on a Broadway musical that flopped, he recorded a "comeback" album, You're the One, that was nice but nowhere near his best work. Paul Simon comes as close as any so far to beating "the theory" ... he'd have done if for sure if his 50s and 60s weren't so devoid of interest. Still, depending on how you rate Graceland compared to Bookends, you might at least make a case that Paul Simon's peak came when he was 45 years old, which we have seen is pretty remarkable in itself.