Can 17 yr. old do excursion with 19 yr. old brother?

[AnnaS]

I am curious since there are some excursions in the West Coast cruise/ Mexican ports that my sons might want to do and us parents might not. Will they allow my 17 yr. old to go out on an excursion with his 19 yr. old brother or does my older son need to be 21? Or is a parent needed? Before we make any tentative plans, we should know if there is a rule. Thanks in advance.