I don't consider it old fashioned, but I really don't hear it regularly. I think people say 'loads of energy', 'energetic', etc.

I have been studying music for about 31 years and from time to time people around me use this word amongst others to help another understand what is needed from them musically. By the way, I study jazz and concert music!

I quite like this word. energetic and passionate are quite strong emotive words, but pep sounds bright and fun but not over the top. it often used in the phrase "to give someone a pep talk" (give someone a little talk to encourage them or cheer them up) or another way could be "this room is looking a little old and faded it need a little pep up"