help needed on waiting list spreadsheet

I am using a range of data to display age at a certain date.
The formula i am using is: =DATEDIF(C3,R2,"m")/12 where C3 is a date of brith and R2 is the current date.
My problem is that when I copy/paste the formula across the range, the number after C and R changes incrementally (so it becomes
=DATEDIF(C3,R2,"m")/12
=DATEDIF(C4,R3,"m")/12
=DATEDIF(C5,R4,"m")/12)

and I want it to remain constant like this:

=DATEDIF(C4,R2,"m")/12
=DATEDIF(C5,R2,"m")/12
=DATEDIF(C6,R2,"m")/12
etc

Can you help?