Does anyone have any actual proof that 'Horde is older and more experienced players' and 'Alliance is young noobs'?
As far as I know you can not tell the actual age of the people playing.  Plus logic would dictate that if anything, the Horde would be full of the younger players.  If we are stereotyping, kids are better at video games, have better hand eye coordination and thus would be better at WoW which would mean if Horde players are better at PvP it is because they are kids who have the ability and the time to play more than older people with jobs and responsibilities.
As far as the original post is concerned:  Pick whatever class and race most appeals to you.  That is what will determine how much fun you have playing.