Here's what I would do.
Put it to the exit event of your numeric field.
function calculateAge(aDate) {
var oDateOfBirth = new Date(aDate[0], aDate[1] - 1, aDate[2]),
iDiff, oAge,
oDate = new Date(); // get current date
oDate.setHours(0, 0, 0); // reset hours, minutes and seconds
oDate.setMilliseconds(0); // reset milliseconds
iDiff = parseInt(oDate.valueOf(), 10) - oDateOfBirth.getTime() - 1,
dAge = new Date(iDiff);
return Math.abs(dAge.getUTCFullYear() - 1970);
}
var cValue = this.rawValue,
aDate = [cValue.substring(4,6), cValue.substring(2,4), cValue.substring(0,2)],
nAge = calculateAge(aDate);
Textfield1.rawValue = nAge; // show age in Textfield1