Linking Two Fields From Two Different Tables

I have two different tables. Table1 contains information such as the name of a patient, which is the primary key, date of birth etc (fields that will not change). Table2, linked by the subject id, contains information about tests that they may have gone through. A form has been made using these two tables, in which table1 is that main form, and table2 is a subform. I want to add a field in table2 called "CurrentAge", which will use the date of birth specified in table1, to calculate their current age, and would like it to be displayed in the subform.