I'm pretty sure you are correct, it was an age discrimination thing, I can't remember the exact year it changed though.

The original MGP age limit of 40 changed sometime around 1998 when they stretched it out to something like 45 for the main classes and I think 48 for the classic's, then they scrapped the whole age limit thing a few years afterwards, probably around 2002, but i'm struggling to remember as age plays tricks with the memory