Query that compares dates
Most likely because the expression 1/10/1971 is interpreted as a numeric expression, i.e. 1 divided by 10 divided by 1971 which is 0 (with integer division rounding values down). Telling Access that this is a date can be done using quotes (not really recommended) or by using a date function, like this:

IIf([DOB]&lt;DateSerial(1971, 10, 1), "Mature", "Not mature")


The above DateSerial() VBA function will return October 1, 1971. The parameters are (Year, Month, Day).

Also, the following will work (thanks, @ypercube!)

IIf([DOB]&lt;#1971-10-01#, "Mature", "Not mature")