I was always lead to believe that the age limit at the Manx was to make people move onto the TT. The Manx age limit was 40 and I'm fairly sure the TT was 55. In 92 the age limit was 40 my dad was sponsoring Mick Robinson and that was his last year before he went over the age limit and moved onto the classics. Three years later the age limit was moved up to 45 and Mick got another couple of years on a modern bike. When the age limit was completely removed I'm not sure.
There never was an age limit for the classics in the same way as former TT riders were allowed in the classics right from the start.