In HTML you can create a drop down list for a user to choose from by creating the list and then putting the options in. Place this just before your submit button:
<select name="age"> <option value="new">under 18 years</option> <option value="fresh">18 to 29 years</option> <option value="mature">30 to 59 years</option> <option value="vintage">60 or over</option> </select>
Try it. Note that the name goes in the select element but the value comes from the option element which was clicked on.
The first item shows in the list of options before the user chooses one. To avoid this add a new option at the top with no text inside the tags and there will now be a blank options at the top. Delete the value attribute completely. Try the page again.
You can have one option pre-selected by default by adding a selected attribute to just that option. Try it for 18 to 29.