I seem to remember Pierre Latour winning a TDF white jersey in his last year of eligibility while finishing outside of the top ten just three years ago. It really should be a temporary problem, although I wouldn't mind the classification being scrapped altogether tbh.
I don't think its a temporary thing, riders are just training and working much more professionally in the junior ranks and if that's the case from a very young age there is nothing much holding back a 20 year old from performing better than someone with 30.
I'm all for a old riders classification though.