Datediff will give you the number of Years, but does have an issue. For instance someone who is born October 30, 1972 will show as being 31 years old if you use the formula Datediff("yyyy",#10/30/1972#,Date()).
In actuality they are only 30 (and 6 months). below are two formulas that could be used to accomidate this type of discrepency.
'This uses the datediff and a boolean expresion (Return is 0 for False and -1 for True) If the birthdate has not happened yet this year it subtracts a year from the calc.
Datediff("yyyy",#10/30/1972#,Date()) + (#10/30#>Date())
'This one uses the Integer division. It calculates the number of Months that has occured and divides by the number of months in the year.
Datediff("m",#10/30/1972#,Date())\12