The average age for Luxy will the the youngest for the 4 you listed. You'll find a lot of 18-22 year olds there, or even highschool kids using fake IDs. As for Room 18 and Myst, the average age will be higher, around 22-30's+. I have only been to Brass Monkey a few times, and that was quite a while ago so I don't really remember what it was like there.
You can also consider Spark. It's at the base of Taipei 101. Of the clubs that you mentioned, I would recommend Myst. It's the largest and is the most happening club in Taipei right now. Spark is also great, but be prepared to be squeezed. It's extremely packed because the club is so small. Room 18 was not bad until they renovated the place, making the dance floor really small. Or if you got cash to spare, you can just bounce around Room 18, Myst, Spark, and all the clubs in the area since they're all within 5 minutes walking distance from each other. There's the ones already mentioned plus InHouse, Brown Sugar, Quube (in Le Meridien), Marquee (lots of foreigners), Woobar (W Hotel), Stream, and Uptown. I would avoid Lava and Babe 18. Those are the all-you-can-drink clubs filled with a really young crowd.
Oh a side note - if you guys don't plan on getting a table at Myst, you can enter without cover by finding out the passcode for the night. Hope this helps.