I'm looking to do a few poker runs this year with my 17 year old son and some of his friends. When I started to register for the first run, the liability waiver says that everyone in the boat has to be at least 18 years old. Is this a standard age restriction or do most runs allow teenagers??

Seems to depend on the organizers who are promoting or where its at more than anything. It may be due to insurance and permit reasons. Im sure you could contact the organizer and find out why.
Our son who is 17 now has done a couple of the FPC Key west runs with us as well as Tickfaw which we have done 3 times. Only problem with him being under age is most bars/cardstops wont let minors in certain areas for certain events which can be a pain but we put our parent hat back on make it work so he has a good time and a fun experience as well.