I'm currently finishing up my Master's in Fisheries, so deal with marine biology-type stuff a lot (though I currently work on steelhead). While going back to school to pursue a marine biology degree isn't really an age limited thing (more on that later), remember that there are a lot of people who go into marine biology. In particular, there are a lot of young folks who go into it and are willing to work for nothing, which is good because there are a ton of them doing it. What isn't so good is that a lot of them wind up dropping out of marine biology because there are a ton of young folks doing it who are willing to work for nothing, thus keeping wages low and competition high. The only reason I'm happily staying in the business is because I have specific mapping skills and technical experience that is pretty valuable, so finding a decent job isn't hard (but I don't work on anything sexy).
Just by way of example, my uncle has a Master's in marine biology and currently is an IHOP manager. The guy who did most of the framing on my folks' garage also has a Master's in marine biology.
As far as travel goes, it depends on what you're willing to put up with. Everyone wants to work on coral reefs or whales (even me, deep in my heart), so most BS degree-level opportunities wind up being unpaid and hard work. I know some folks from a lab at school who work the Bahamas and Hawaii every summer (diving all summer doing fish surveys), and they love it, but there's never any shortage of highly qualified warm bodies to throw in the water for very little money (or charging to let them do the work; sketchy but it's done).
That said, I have seen older folks come through school with me and do fine. They tend to be the irritating ones in class who want to ask very long, rambling questions that distract the professor from the material at hand, but I think they pass. If you decide to do this, look for a school that caters to older/returning students. For example, University of Oregon is a bad place for such students as it predominantly serves young undergrads/grad students, while Portland State is considered the "old school" because it has so many older/returning students (largely a commuter school).
Not to be too depressing, but it's a challenging profession succeed in, even for young people. I'd suggest finding your local public aquarium and being a volunteer diver there for six months or so; it's a great way to get involved in that sort of thing, and can give you some idea of what/who you're dealing with.
