practicality

Some communities and moderators will let you in with an e-mailed age statement if you're not comfortable with a year of birth listed in your profile (since some people, especially outside of North America, have very real concerns about identity theft using their date of birth-- this is something that came up with British LJ users during the whole "list your year of birth to make our stats more accurate!" debacle). Not all of them, by any means, but sometimes it's worth a try to ask.