Peter's been hanging out with Alba since at least this summer, showing her off at some fancy parties in the Hamptons. We know -- age is just a number, but this is kinda wild.
Peter and Christie's daughter, model Sailor Brinkley-Cook, is the same age as Alba ... her new stepmom-to-be.
Alba is currently a student at Tulane University in New Orleans, and according to her social media, she'll be graduating in 2021. As for the other important date in her life -- no word on when the happy couple's planning to walk down the aisle.