I think Gov't Mule has a fairly age-diverse fanbase... yeah, there's a lot of young fans, but I know plenty "middle-aged and older" who are big fans too. Mule is connected and has played with so many other artists that I think a wide range of people have been drawn in over the years. When I saw them a few months ago I remember the audience being pretty diverse, if not leaning toward older. In fact I struck up a conversation with a middle-aged(?) guy wearing a Joe Bonamassa t-shirt. 
On the JamBase site, which I'd say is mostly frequented by young people, and where Gov't Mule is HUGE, I once posted about Joe Bonamassa in a forum thread titled something like "What's the last band/artisit that knocked you off your feet?" It got no replies except for a guy who agreed with me about Joe, but told me I was wasting my breath (so to speak), because he'd tried to turn that crowd onto blues before and nobody cared. Made me sad actually... and flabbergasted, since blues is pretty much at the root of everything they all like! Keep in mind though, only a fraction of Mule's fans are JamBase members, and most members don't bother using the forum, so this experience may be misleading. 
I just remembered, I actually saw a thread about Back Door Slam on JamBase the other day... don't remember if I read it though... but anyway they're getting attention there due to touring with Gov't Mule, would be my guess.
Generally speaking, you're right about the younger generation not being into blues, which is sad. However, there is a segment of the young population who knows good music and craves the blues... my son and all his friends are in that group, thank god (yeah, I raised him right, lol)... they know they are unusual, and proud of it too!
Well anyway, I for one think these two bands make a great touring pair... probably apt to make new fans for BOTH bands, don't ya think? 
--Vik 
