Year - Drop Down Box

Basically what im trying to do is create a dynamic drop down box that changes its output each year.

It is a drop down box that displays years, but i need it to be always a minimum of 18 years minimum from the current year e.g as its 2004, it needs to display 1986 as its minimum year and upwards to 40 years from 2004, because the type of people im looking for need to be over 18, and no more than 40... And this is within a form, so the users choice of years is collected on submit.