Whether you're a youth or adult, if you're participating in a troop activity, you need one.
Here's all the information on the Health and Medical Forms.
In a nutshell:
Parts A and B - Parent / Guardian Signature Required. Good for most activities except High Adventure and Summer Camp.
Part C - Physician Signature Required. Needed for Summer Camp and High Adventure.
Part D - Physician Signature Required. Needed for High Adventure.