Not all alumnae associations have an average age of 50 or so, though I must say I know LOTS of "older" alumnae and they inspire me to stay involved!!!! My assoc has the average age of 25! I'm an older one at 28! Eesh! It really depends on what you are looking for. The fact that you researched is good. Don't get discouraged....depending on your location, there are probably many more young alum out there that have not "discovered" the association yet.
If you are in college, check out the college chapters. What do you have to lose? I mean, really, everyone on here can load you up with the positives or the "hard truths," but not many will know your school.....so the advice won't always hold water. Check it out. Have fun doing it and just see what happens!