As you say, it's a problem to avoid if we can. However, we can't, so - here's my personal opinion. I'd avoid 'old', 'elderly' and 'ag(e)ing; Even 'senior (citizens)' smacks of old age to me. 'Mature' seems as good as any for an adjective. As for a noun 'over-50s' seems acceptable. As the barrier is set at 50, when many people are still working and very active, it is a nicely inclusive term.
Student or Learner