I'm installing a breaker panel in a detached garage, What is the minimum mounting height for the panel?

There is no minimum, here is where common sense prevails. Do not put the panel so low that splashing water or it can be subjected to physical damage without reasonable protection.

I would always think of future owners and kids -- I like to mount them high.-- Savage

Have I read that 6' is the maximum?

I always put the top of the panel at 6', customary in commercial, If it has meter socket check with utility company for height to center of socket (jaws)