As mentioned with WoW, there are a lot of kiddies, but due to the IP involved and its emergance as one of the first really mass market MMORPGs there are a lot of adults playing too, just find a guild with a more mature player base, rather than one averaging around 16yrs and you should be fine
My main character's guild has people ranging from 16+ to the best part of 60 yrs old, so WoW has picked up a much larger demographic than the stereotype.
Might also be worth looking around the forums if you consider it, as despite the huge amount of whine, you can sometimes pickup good information on classes, and which servers would be good to go to.
WoW has its problems, but its one of the few games I've played, and despite having cut down time due to work and not wanting to lose my life, had a few breaks etc, Im still playing after 2 1/2 years, so there is plenty of content, if you look at the overall experience - levelling/raiding/doing crazy stuff for fun/pvp/social...a lot of people focus on one aspect after a while, and thats often when their enjoyments soon starts to begin to suffer.
As mentioned, next patch, they're improving the lvl 1-60 grinding process by reducing xp needed, buffing the loot and making solo quests in the old content easier (ie elite mobs---> standard mobs after patch2.3) so you can catch up to the Burning Crusade expansion quicker, might be worth trying then.