Is the date of birth part of the ID like in the Florida, Illinois and Wisconsin Drivers Licenses or are the DOBs stored in a tab delineated text file or an SQL database.
If in the ID just do the math, if it is in any text file LC Designer can not do it - look at Acrobat Forms, and if in an SQL database, including MS Access and MS Excel Named areas, you will have to create a data connection and then execute SQL commands to retrieve the data by the ID.