21ST BIRTHDAY CELEBRATION
I'd say that Bricco is young, fun, and fits the "luxury" well enough. The one thing sorely lacking in the North End is a cocktail program. First to market on that front should enjoy a nice little surge.
As to your second point, that's just not true... Here's a short list w/ open tables for 4 at/around 8PM on the 20th.
Bistro du Midi
Clio - definitely luxury, maybe not so young? I've never been.
Del Frisco - my fave of the steakhouse chains esp if they are boys since the waitstaff is dressed rather, uh, playfully. Great bar scene in the area and centrally located.
The list goes on but you get the point, right?
For a 21st birthday (outside of cambridge due to Head of the Charles), my suggestions would include: Masa, Eastern Standard, Island Creek Oyster Bar, Citizen, Towne, Scampo. They each have reliable food and fun, lively young atmospheres. Some may be less luxuarious than others, but I geared my suggestions towards the guests of honor.