This is a countdown until the next birthday (formula duration field), but the variables are what you need:
var number Age=(Year(Today())-Year([DOB])-1)
+If(Month(Today())>Month([DOB]),1,0)
+If(Month(Today())=Month([DOB]) and Day(Today())>=Day([DOB]),1,0);
var date Birthday=AdjustYear([DOB], $Age+1);
$Birthday-Today()
You can pull the Age, Or the next birthday, or the countdown.