Custom Attribute Date Picker

Hi all! We have created a custom attribute to capture date of birth and selected "Date Picker" as the attribute type. When using the form on the Client Portal, the year only goes back to 2012. If you type in an earlier year, the list will reorient itself and additional years will appear, but we worry that customers will rely solely on the date-picker and might not think to just type in their birth year. Is there anyway to exclude future years and instead offer more past years? Thank you!