I think they do skew older. Message Boards skew younger. But, even on this board, most of the most serious GA collectors are over 50, many in their 60s. Think about the guys whose collections we talk the most about, guys like the Dentist, Geppi, Fischler, Halperin, Bangzoom, Moondog, etc., and it quickly becomes apparent all of them started collecting in the 60s. And they are just the tip of the iceberg, because there are a lot of deep GA collections across the country in the hands of guys who started back then. Personally, I think that while their plenty of examples of younger folks with GA collections, if you had a list of the best collections, the vast majority would be owned by older guys. The young waiting in the wings are going to either inherit a great collection (see Verzyl's heirs or, in the future, Blissard) or are going to have the opportunity to buy a number of great collections over the next 20 years. I really don't think inventory is going to be much of an issue for the GA market.
P.S. I'm 55, but have had the advantage of my father's childhood collection he gave me in 1979 that he amassed in the late 40s and early 50s (he's 82).