Mansion: Check for ID if you go here
Vinyl: 18-25ish, retro music, carpeted floor
Whiskey: The general place to go for most people
Roadhouse: you got your young ones here, but also the 20-30 crowd
Hifi: if you're a hipster, go here.
Back alley: If you like underage girls, cheap drinks, and one dirty ass club, this is for you
Republik/Klub OMFG: If you're a rowdy hipster/electro lover (fridays only rly), go here.
as far as pubs, I don't go to alot of them, but drum&monkey is ALWAYS busy, there's a good amount of younger people there too. Melrose is also a SUPER busy one, more "upscale."
I covered the main stuff but if you tell me more about what you're looking for, what area, dudes, etc. I can maybe help you more.