Is there no way to go into your profile to update it?
You also might want to try contacting them and seeing if there's anything they can do to change your birthday to the correct date if there is no way for you to do it yourself.
I highly doubt if it's just a simple error like this they'll freeze your account and money (unless you're not of age to their terms and conditions or something like that)
Nope, I can't update it on my profile :\
The problem is that when I opened the account with them I wasn't 18 yet (I'm 18 now though). There's a point in terms and conditions that says:
"4.2. You must be 18 years or older to use our services and by opening a Moneybookers Account you declare that you are 18 years or older. This does not apply to products for which we set a different age limit. We may require at any time that you provide evidence of your age."
I declared I was over 18 when I opened the account, but I wasn't.