Maybe to some of us it isn't (it certainly isn't to me), but some are very secure (or paranoid) with every aspect of their online information to a point that it becomes a worry that they're not safe at all.
It's sort of like how some people I know on Yahoo IM are asking me if I use MSN IM because they don't want to use Yahoo anymore; they say it displays their real name in IM boxes, but I've never seen their real name displayed, so I suspect the problem is either one-sided and they have nothing to worry about, or this is something new.
Some people see a huge security risk, and others see a triviality with listed birthdays.
Many people are lying about their age just to be here anyway or are passing themselves as older or younger than they really are (or even passing themselves as the other gender), so the birthday requirement (and optional gender profile) is already flawed anyway. The people we think are certain ages may not be, and the people closest to us may be lying to us. Some people probably even thrive by creating a completely different identity and personality online because they either like pretending or are that insecure.
If some people feel keeping even their birthday numbers a secret is vital to their security in a close-knit community such as this, then it's their right to feel that way, plus, as people have said, they can simply lie, since nothing is stopping them from doing that and theres no way we can prove they're of legal age anyway.
I think it's trivial, myself, mainly because if you have at least your vital information secure, people knowing the small stuff isn't linkable to the critical information at all, since you have that sealed off.
