I am thinking u can do with javascirpt. just you have take the value crom Birthday and check it, then popup the desire msg.
function CalculateAge(birthday, ondate) {
// if ondate is not specified consider today's date
if (ondate == null) { ondate = new Date(); }
// if the supplied date is before the birthday returns 0
if (ondate < birthday) { return 0; }
var age = ondate.getFullYear() - birthday.getFullYear();
if (birthday.getMonth() > ondate.getMonth() || (birthday.getMonth() == ondate.getMonth() && birthday.getDate() > ondate.getDate())) { age--; }
if(age>18)
{
alert('msg');
}
}
thanks
regard
Sangram
microsoft blog