Queries :: End Of Year Age Based On A Date

I have a date of service [DOS] and a date of birth [Birth Date]

I'm trying to calculate the age at the last day of the date of service year not the current year.

Example:
[Birth Date] = 6/25/1993
[DOS] = 10/18/2013
Age at the last day of the date of service year (12/31/2013) = 20

How to do this. I tried

Code:
DateDiff("yyyy",[BIRTH DATE],[DOS])+Int(Format([DOS],"mmdd")<Format([BIRTH DATE],"mmdd"))

but this does not work correctly.