Use Calculated field and Datediff function to determine the number of years between two dates. Here are some examples:

Number of years between two dates: Datediff(year,datefield1,datefield2)

Number of months between two dates: Datediff(month,datefield1,datefield2)

Number of days between two dates:Datediff(day,datefield1,datefield2)

More information about all the parameters and options of DateDiff  function is available at http://msdn.microsoft.com/en-us/library/ms189794.aspx.