The drop down initially shows a range of 20 years. Once you select an earlier year, the range changes to display earlier years with the year you choose to be the middle value. That is why it shows 1999-2019 initially, since it uses today's date as a default.
to display the date range to be what you want find this block of code in page you've applied the form to:
At this point you have an option of what you can do. After the line:
You can add either of these lines in:
The first one is a hardcoded date range, whereas the second one will change the date range as time progresses.
Here's a reference for if you want to change anything else on the Date Field: