You got me curious. I just went through my burner email list and added up the ages of the folk for whom I knew their ages. That ended up being 40 folks and the average age is: 31.4. I then threw my age in and I bumped it up to 31.9. 31 plus or minus one is pretty close to my impression of the average age at Burning Man. I would say the main mass is between 25 and 35 with enough young folks and old folks to off-set each other.
My first Burn was in 02 and I was 49 and I was there for about 5-6 hours before I saw anyone that I thought was near my age. My first thought was 'Thank god, another old fart'. After a while I saw and met quite a few folks my age and older. I also started to discover that by and large nobody cared.
I ended up doing things with people in their 20's as often I did with any older age group. Now days my social group consists of the 40 people (plus others) mentioned above.
One thing I noticed at Burning Man and have observed since, there are Creepy Guys out there and the worst are Creepy OLD Guys, don't be one. Wear costumes, ALWAYS ask before taking pictures, DON"T hug or ask for hugs from women you don't know, don't touch, don't keep hanging around if she makes it apparent that she is not interested, don't lurk, that is, follow girls around hoping to get up the courage to say something. Or do the don'ts, my current sig is based on young guys doing 'don'ts' and two of the girls hanging on to me at various times to get the 'creepy guy' to go away; makes my life better.
My grandfather tried to raise me as a Southern gentleman, that means that I can be a real SOB some of the time.