It goes in the Age field.
You might want to use:
var dobField = "Dob_Field"; // name of dob field
var dobFormat = "d-mmm-yyyy"; // format for dob field
event.value = "";
dobValue = this.getField(dobField).value;
if (dobValue == null) app.alert("dob field not found");
if (dobValue != "" && dobValue != null) {
oDob = util.scand(dobFormat, dobValue);
if(oDob == null) app.alert("dob conversion error");
oNow = new Date();
if(oNow == null) app.alert("now conversion error");
if(oDob != null && oNow != null) {
// set date objects to midnight - start of day
oDob.setYear(oDob.getFullYear(), oDob.getMonth(), oDob.getDate(), 0, 0, 0, 0);
oNow.setYear(oNow.getFullYear(), oNow.getMonth(), oNow.getDate(), 0, 0, 0, 0);
// compute age
age = oNow.getFullYear() - oDob.getFullYear();
// adjust age if today's month and date before dob month and date
if (util.printd("mmdd", oNow) < util.printd("mmdd", oDob) )
age--;
event.value = age;
} // end dob and now not null
} // end dob not empty not null
It includes checks for wrong field name and date conversion errors. I would also recommend using the 4 digit year so you do got a negative age for DOBs prior to 1-Jan- 1950.