We started (wife and I) doing pretty serious sailing around mid-20's. We got very comfortable with sailing on a 320, then had a kiddo, and put him on the boat at age 5 days. By 29 or 30, we were living aboard a Catalina 380 and cruising as much as possible. Now, 6 years later, we have a Caalina 400 and are about to embark out again, hopefully for a lot longer and further this time with the improvement in technology.
All that being said, let me tell you that your perception of the marinas being primarily an older generation is quite true. If you are under 50, I would say, you will be in the minority. (PS, the exception to the older generation is California, especially Southern, where there seems to me to be a lot younger croud. I wonder if others have seen this like we have? Are there other areas?). This might seem bad, and there is the obvious generation gap, but it really is not bad at all. The 50's and 60's year old we met were as cheerful and vibrant as anyone in their 20's and 30's. Age, race, sex, nationality... you will not find it a negatively defining characteristic. In fact, we often seek out other flags. The absolute funniest people we have ever met were Jamaicans and Australians, especially the Aussies! The most family oriented were probably Brazilians. Canadians were often the most open and friendliest and Americans often the quickest to help with problems. With a few exceptions, you will "click" really well with all of them. Many may want to keep your kids (if you have them), and you will find every weekend another boat party, in general. I would not say a boat party is a drinking party, big exception. We would get together and scream at each other over a friendly card game, or the proper way to grill fish, some politics (very little, suprisingly), and a lot of talk about neat places to cruise. You will become a close family and will not realize they are older. We did meet a young couple (mid 30s) that became our best friends in the world (and still are), but the younger generation, as it was put, is more far and inbetween. Family's cruising is even further from the norm.
Most of the people you will meet are very intelligent, educated, and were very successful in their pre-boating life. But when they hit the water and were on a boat, all those frustrations were left behind and you will see them for what they really are: Just great people. Funny, I am not sure I know what most of them did to earn a living?? We rarely talked about it. Bird identification and boat troubleshooting were more likely topics.
I will say that the reason the younger generation is absent is not a lack of adventure or being too tied down. Boating and cruising is very expensive, and not getting any cheaper. Most of the people are older because they are retired or spent much of their life doing very well and now have the money to go cruising. I think that is what stops younger generations. Many may ask how we did it (and are about to do it again) and I will tell you that we have been very bleesed with our careers and we also have fairly unique positions that allow us to work remotely anywhere with an internet connection.
All that being said, I agree with Cam and am in the minority: Save up and go versus just jumping out there on a wing and a prayer (financially speaking). Take your boating budget and double it. Ask PBeezer about how quickly things can go wrong and get expensive! Also, I have painted a Cruising World type picture of how "rosie" cruising and living aboard is... it is not like that all the time. Life is still life, whether from a boat, a house, or a camping trailer. In fact, I will say it is much easier from a house than a boat and the frustrations are considerably less. Want an example? Next time a really big storm rolls through, sail out to meet it. You will get stuck in one sooner or later. Try sitting down below in your boat for a weekend and hardly leaving it.... fronts come through and restrict your ability to do anything.
Tom Neale wrote a book called All in the Same Boat. It has been said that it has a lot of negative connotations... well, yes, maybe it does. However, it paints a much more realistic picture of what it is like in my experience. I would highly reccomend it. Having the right mindset and expectaions are better than getting surprised around the first marker.
It is not always fun. It is not always boat parties. The younger person(s) is more the exception than the rule. Still, it is worth it and you will find that the generation gap is not a big deal. Save your money. Read, read, read. Then make the pluge and find out for yourself. You will be glad you did.