The ultimate source is in the odoo/odoo/addons/web/static/lib/jquery.ui/js/jquery-ui-1.9.1.custom.js which sets the yearRange to be c-10:c+10, i.e. current - 10 years to current + 10 years.  That's why you are seeing years from 2005 to 2025 this year.  The DatePicker instance, I think, is instantiated in the odoo/odoo/addons/web/static/src/js/view_form.js file (search for the start method of instance.web.DateTimeWidget).  You can either change it from that file (add yearRange in setDefaults to your satisfaction) or override the start method of instance.web.DateTimeWidget.