If we do an age limit it should be scientifically defined, something like the statistical average age at which mental decline due to old age has set in. Wording it like that will allow the exact age to be flexible with the times if say longevity increases or a generation is born into such harsh environmental conditions that it lowers the average cognitive ability at a younger age than expected.
Mental decline due to old age presently begins to set in noticeably in the mid to late 60s.
Plus, it's better than the present system where authority and paying jobs are held onto for dear life by zombified boomers out of what can only be assumed as sheer spite for the up and coming.
I think just having a cutoff works better, otherwise the test will be constantly scrutinized and attacked and neutered by olds trying to break their way into holding onto power forever in spite of the greater good.