look at a profiles on there and on here, do some maths and get a median age.

I'd discount the people who say they are 100+ plus though. That might sink your battleship.

and measure instances of:
txt speak
gratuitous use of emoticons
comic sans

that should give you an idea of the amount chumps following said band.

Stick your head in the oven at Gas Mark 8 for 30-40 minutes, turning once.

http://keef.co.nr/?p=lastfm-1

Just stick in the artist name and it does it for you, from last.fm.
Quite slow, but at least it's automatic... doesn't work for all bands though - I just tried Enter Shikari cos I wanted to laugh at how young they all were, but it wasn't having any of it.
Yo La Tengo fans, though, are apparently 23 on average, whilst P!ATD fans are 17. LOLZ, etc. Tells you gender as well - eg, nearly all Britney fans seem to be male.

Of course, it's somewhat skewed by the demographics of last.fm users in general...

Have a huge database called ViceVersa. Are this band on any of their imprint/subsiduary labels?

you should just guess - it cant be that hard.

That way you can usual 'realistic' evidence to back it all up?

ie:

The nme.com audience is made up of key demographic groups that can be difficult to target through other media: -
65% Male*
50% 16-24*
23% 25-34*
79% ABC1*