I'm really interested in training Muay Thai, but I really want to avoid any brain trauma. If I spar only lightly (to the head) once a week, will I be able to train for years while staying healthy as I age?
There is an old russian wisdom - "who don't take risks would not drink champaign".
If you are aware of brain trauma, maybe it would be better to pick something other - without intensive full-contact punches to the head.
Muay-Thai (along with boxing and KB) do allow strict strikes to the head with arms. And that work is usually very intensive. And, as long as head is a vulnerable part of body, most of punches are aimed to it. For me, working in M-T class without full head punching is like sitting in a car without starting engine. You may learn what is car as whole, but is it enough?
Maybe you should pick up Taekwondo, or Karate - there are plenty of kicks and punches, which are, along with its effectiveness, just beautiful.)
If you still decide to get in - beware of "light sparring" - always be prepared that your sparring turn into a fight-till-ko, because both of you may engage and start hitting hard.