According to official MotoGP statistician Dr. Martin Raines, there has only been one older 500cc/MotoGP debutant than the 37-year old Jamie Hacking. That was long-time Aprilia tester Marcelino Lucchi in 1996 on the Aprilia 400cc v-twin at 39 years of age.

However, Lucchi had ridden in the 250s for many seasons, both as a regular entrant and regular wild card, so we can safely say that Hacking is the oldest Grand Prix debutant on record.