Bug 12342 has a fix for the staff interface (http://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=12342) for date picker available range. This is the same fix but for the OPAC.
This was originally part of bug 12342 but was moved out to a separate feature. When new patrons are added the default selected date is the current date minus one day. I assume we do not register people on the day that they are born. This patch changes the default to -15 years, assuming the average patron entry age is around that age.
Did you forget to apply the patch?
Both the datepicker for date of birth in staff and OPAC go back a 100 years by default now. Both preselect to today's date. I am not sure if going back 15 years by default would really be helpful with the new longer year select and the option to type the date in directly? It might raise questions instead for being arbitrary.
The flatpickr year selector works differently than our old one. Instead of a (limited) list of years, you can increment/decrement by 1 OR enter the year into the field. I think this renders this bug invalid for flatpickr.