The answer to this is dead simple.
Let's take a developer who has made it a lifelong career. They started post college at age 22. This means they began working in games in 1996 or before and are still at it. The number of game developer from this time is miniscule compared to today, which itself might result in 9% or less. Additionally, games being the difficult industry it is, there are any number of reasons for someone to leave the industry well before age discrimination becomes a factor.
Or the case of a developer moving over later in their careers. Surely this is an extremely rare occurrence as the pay and ability to do things like raise a family will keep them in more traditional (stable) jobs.
There aren't many 50+ developers (and that number is turning into 55+ way too fast for me) today because there just weren't that many 20 year old developers in the 90's.