You might like to take a look at Age.zip in my public databases folder at:
https://onedrive.live.com/?cid=44CC60D7FEA42912&id=44CC60D7FEA42912!169
This little demo file illustrates the use of a number of age related functions, including a GetAge function which you can paste into a standard module in your database. You can then easily create an expression for conditional formatting. You can see how it would work by entering some literal values into the immediate window, Firstly for a date of birth for someone over 21 on 15 July in the current year:
Birthday = #2000-07-04#
? GetAge(Birthday,True,DateSerial(Year(Date()),7,15))<=21
False
If we then change the value of Birthday so that the age will be under 21:
Birthday = #2002-07-04#
? GetAge(Birthday,True,DateSerial(Year(Date()),7,15))<=21
True