Not stupid, I wrote that in a hurry, should have explained better.
DateDiff is a VBScript date function.
(start a copy from 4guysfromrolla)
DateDiff returns the difference in intervals between two dates. The interval can be one of the following settings:
"yyyy" Year
"q" Quarter
"m" Month
"y" Day of Year
"d" Day
"w" Weekday
"ww" Week of Year
"h" Hour
"n" Minute
"s" Second
So, DateDiff("d",#10/5/98#,#10/8/98#) would return 3, since there are three days between 10/8/98 and 10/5/98.
(end copy)
you need DateDiff("yyyy", DOB, Today), if return 18 then the user already have 18.
Off course you will need to change manually when a user signs with less than 18 but then complete 18 years old, although you probably can do some automatic way to do that.