The cadets that have been placed with us over the last 15+ years have certainly included people in their "Thirties." They are not in the majority, but that is simply because the majority of trainees (as you say) tend to be Twenties and some late teens. It has never (to my knowledge) ever been an issue on any level. That notwithstanding, equality regulations in various EC states would likely preclude age being a sole selection for disqualification.
Obviously, different parts of the world may have their own selective criteria whereby age limits form part of that criteria, but I never seen any evidence in my own sphere of observation that would suggest you should be concerned on this score.