You are zero years old

The mistake

The date-of-birth field is initialised to the current date instead of being left empty, so any customer who submits the form without editing it reports a birth date of today and the age check computes an age of zero, failing the over-18 gate for everyone. Changing the default to 1900-01-01 swaps one wrong default for another: now every unedited submission reports an age of 126, which clears the age gate but also trips the over-60s discount rule. The fix is to leave the field blank and require an explicit value, rather than pre-filling it with a date that is never true.