Under 21?
Well, sucks to be you then. Especially if you like live music. Whether you come from the teenage drinking paradises of Canada or just the more lenient of these United States (say, Texas), you'll no doubt be surprised by the old-timey prohibitions of the Washington State Liquor Control Board. Of primary concern to these octogenarian codgers is that kids and booze are never in the same room with music. Family restaurant? Then your dad (or uncle or older friend) can get tanked with you on his lap for all they care. Music venue? You're fucked.
While it's now legal for bars to host all-ages shows in Seattle—thanks to a repeal of the city's odious Teen Dance Ordinance some years ago—it's still not financially attractive for most places to do so (typically, it's only done at the request of bands). So for those of you in the absurdly frustrating 18- to 20-year-old demographic there are still only a handful of reliable sources for music and nightlife:
• The Vera Project, Seattle's well-loved, city-supported all-ages venue at Seattle Center, has regular shows ranging from punk to indie rock to local hiphop, and it's a great place to get involved with the local music community (plus the nonprofit is always looking for volunteers).
• Gallery 1412 is a collectively owned, donation-based music and arts venue dedicated to more avant-garde artists and bands.
• Seattlediy.com (DIY, for the uninitiated, refers to the guiding punk principle of "do it yourself") is a great resource for those looking to connect with the city's underground all-ages music community. By far, the most vital and rewarding all-ages shows happening in Seattle at any given time are the ones happening in punk house basements, and seattlediy is a great place to meet the kids putting those shows on and get involved.
• Club Pop is a monthly indie concert/dance party hosted by Antarctic Records' Michael Yuasa at Chop Suey. It's the best regular night going for those over 18 but under 21, featuring electro and rock acts such as Glass Candy, Chromatics, Le Castle Vania, Tommie Sunshine, Holy Ghost Revival, and others, as well as some of the best-looking club kids in the city.
There are also some venues that tend to throw all-ages shows more than others, and among those are Chop Suey, Neumo's, the Crocodile, El Corazón, and the Showbox. The Stranger always makes a note of all-ages shows, so just keep your eyes out. There are regular festivals like the Capitol Hill Block Party (last weekend of July on Capitol Hill), Sasquatch (Memorial Day weekend at the Gorge), and Bumbershoot (Labor Day weekend at Seattle Center) that offer high concentrations of music for all ages. And there are occasional one-off shows at the UW or other odd locations that are open to the under-21 crowd.
But really, aren't you old enough to get a fake ID already? I was never connected enough to get one, so I couldn't tell you where to pick one up, but I'm sure you can figure it out. And once you do—and it had better be a good one—may I recommend:
• Pony, a temporary gay dive bar occupying the space that housed the first incarnation of Seattle's beloved punk mecca, the Cha Cha Lounge.
• The Cha Cha, now on East Pike Street. Its new location maintains the original's charm while expanding into a bright upstairs cantina and a cool, roomy basement bar.
• The Funhouse, Seattle's "oldest surviving punk club." Like a lot of old punks, it has its share of scars and problems (hearing loss, bad tattoos), but it's a genuinely grimy place to see some genuinely grimy bands.
• The Comet, another historically battered bar that has frequent, frequently kick-ass live shows.
• Havana, the War Room, and the Baltic Room, which all offer regular DJ nights ranging from hiphop to reggae to '80s to electro.
• Nectar, over in Fremont. The club has recently been bringing in some serious live talent thanks to a new booking agent and an upgraded sound system.
• And finally, the Crescent, an upscale karaoke bar that's equally perfect for a romantic date or a night out with the fraternity brothers. 
