How to use tokens in Integer field?
Try the Date module


Install the module
Go to Configuration > Account Settings > Manage Fields tab
Add a date field of type "Date" and widget "Select list"
Hit Save
Deselect every date piece except year
Hit Save field settings.
Check Display on user registration form.
Expand More settings and values
Set starting year and ending year and default value.
Save settings


If you must use a text field, the date field gives you that option as well. You wont be able to set a start or end date, but the module will validate the date is in the proper format.