In my experience, anonymouspeople I've known in your situation were actually hung up on closeness in age to the paramour of a loved one for a fairly specific if often subconscious reason: Are you really going to let a number come between you and your father like this? Does he think this is similar to the babysitter situation maybe he now associates divorce with younger women so he got the urge to date someone young? Either way, let it go. Have you noticed how uncomfortable things are between you? More than a decade earlier, he and my mother had gone through a messy divorce.
Nothing wrong with feeling a little weird.
Ask Michael Cohen: Why Age Matters When Dating
Happily, you don't live near him, so you shouldn't have to put on the Happy Face too often. You keep thinking "This girl and I could have shared a locker! Your dad will do as he damn well pleases regardless of your approval or disapproval, I'm guessing-- because he's gone through plenty of his own personal turmoil over his marriage, his divorce, and his infidelity, and probably doesn't need someone else telling him how he ought to feel and how he should act upon those feelings. I've said this in other threads but I know whereof I speak -- the gods of irony dump a lot of never-saw-it-coming stunning shit on one's head over the course of a lifetime.