You may find the law quaint all you like, if you worked in human resources you would know it's not splitting hairs. If the recent court cases striking down bawdy house laws stick, legislation around employment practices are going to be very relevent soon.mmmm
Here's a link to the Allure webpage where they specifically are looking for women 21 and up only (good for them)
http://www.alluremassage.ca/contact.asp
As to the quaint notion that it's not legal to ask someone their age when applying for a job - I think you're splitting hairs. Any MP owner with half a brain would not only ask how old, but let me see your passport or Driver's license to prove it. Don't have one? There's the door. Good luck to her taking that one to the Human Rights board.
If you read my post carefully you would have noticed that a) I like the idea of +21 in spirit, and b) I didn't expect MPs to stick to employment law. But the day will come when they will, legalization means regulation.