It could be for several reasons.

The most likely are that 1) they got in trouble for selling age restricted products to people not old enough to purchase them and are now under the microscope so they don't take chances. 2) They collect your date of birth to send you promotional shit for your birthday. 3) Age demographics are a huge deal these days, many businesses track age and buying patterns to determine who and what to market to based on aged based buying habits.