I'm pushing 60 and getting my feet back on the floor after an awesome workout the day before is not always easy. Applying oneself to do it again can be hard but not impossible. Before you know it what seemed almost impossible a few months earlier is now too easy. Stay out of the "comfort zone". Challenge yourself to do things just a little bit harder each time and you will grow from the experience of doing what once seemed impossible.

@ op: sounds like a bad instructor without well knowledge on physiology etc.

every person is different. one can't push them all in the same way. for one its no problem, the next one might collapse and i guess he didn't know the anamnesis of every single student.