Conversation so far:
What medical breakthroughs did John Harvey Kellogg make?
Did John Harvey Kellogg invent any new equipment?
John Kellogg invented a hot air bath, vibrating chair, oscillomanipulator, window tent for fresh air, pneumograph to graphically represent respiratory habits, loofah mitt, and apparatus for home sterilization of milk.

Question: how old was he when he started studying medicine?
Self-contained rewrite: How old was John Harvey Kellogg when he started studying medicine?