That's a good question sir, let me just scan some records right quick....
Mike Tyson turned 27 in prison in 1993. He was 24 when he lost to Buster Douglas and after that point in his career he was never considered THE MAN of the division and he never achieved the same success he had prior to prison but since he DID do time perhaps he's not the best case study.
Floyd Patterson turned 27 in 1962 which saw him get demolished by Sonny Liston never to regain his top form again.
Jose Torres turned 27 in 1963 and he lost his first fight that year to Florentino Fernandez, BUT he eventually did better and hit his peak by defeating Pastrano in 1965 (age 29). Consecutive losses to Dick Tiger in 1966 (30) and 1967 (31) began the end of his career .
Joe Frazier turned 27 in 1971 and won the very biggest fight of his career vs Ali and he was never the same fighter after that match. He beat a couple pushovers but Foreman pretty much ended his career in 1973 (Frazier would have been 29).
Jake LaMotta turned 27 in 1948 and he won the middleweight title in 1950 (where he would be 30)and then immediately lost it to SRR never to challenge again losing or drawing the next 4 fights.
Jack Dempsey turned 27 in 1922, only fought 6 times after 1922 and lost to Gene Tunney in 1926 and 1927 when he would have been 31 and 32 respectively.
Aaron Pryor turned 27 in 1982 and in that very year he had the dubious victory over Alexis Arguello and had 5 more bouts until in 1987 at the age of 32 he was TKO'd by Bobby Joe Young.
Ray Mancini turned 27 in 1988, his career was already basically over.
Jeff Fenech turned 27 in 1991, his career slowed to a halt after a SD with Azumah Nelson that year. He then suffered knockout losses to Nelson in the rematch, Calvin Grove, and Philip Holliday.
27 is arbitrary, it's kind of the mean age when the pressure fighters start to lose steam/punishment from using the style adds up on them. Some fighters go for a little longer, some get smashed out earlier depending on level of competition.
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