I have an option group with 2 checkboxes :
MtbAge - option value = 1
RoadAge - option value = 2

The textbox where to display MtbAge/RoadAge is called = currentage

if you select MtbAge it must use
Code:
=Format(DateSerial(Year(Date()),12,31)-[Birthdate],'yy')
to calculate

if you select RoadAge it must use
Code:
=DateDiff("yyyy",[Birthdate],Now())+Int(Format(Now(),"mm
...