It can depend, some of the bigger sites like delta force will probably post vacancies online and give you the application forms to send in. The smaller sites that aren't part of a huge franchise (like the one I work at) you can probably just call. I started really young when I was 15ish because I went their for work experience. After my 2 week placement there the boss just kept me on doing the smaller jobs and shadowing the marshals until I eventually got confident and good enough to marshal my self. I don't think age is a huge thing (although a lot ask for you to be 17) what they're looking for is someone who has authority and can be assertive and safe.
Best bet is to call up and ask. Worst they can say is no... more often than not they'll probably ask you to come in for a trial day. Word of advice on that front, don't be shy and reclusive just because it's your first day. What we look for in someone who's trying out is for them to be loud and confident, scream at some customer to put his mask back down or to lay off shooting his friend in the back and you're sorted. Be confident and safe and they'll probably keep asking you back.
