Of course, what we are actually learning here is the average age of a Home Brewer who is also a member of The Home Brew Forum.
My assumption would be that the Internet Forum world has many more people on board who are under the age of 40 than over, the internet is more of a way of life for younger people than it is for older people.
I'm not saying 40 is old by the way, just that people 40 and under were brought up with computers, once you get above 40 computers were not as common in schools etc..
The BBC Micro, probably the first computer to really start getting into schools was only launched in 1981, with a massive 16kB of RAM.