Yes, Phinn used to wear her hair in pigtail-like things when she was younger. And she didn't dress in her thick Elven robe, either. But, the real question is exactly how old Phinn is in those flashbacks. Truth be told, it could be anywhere in her 20s or 40s or so. "Younger" is, of course, a relative term. To humans, it could mean about 10 or 20 years ago, or longer if the speaker is quite old. To elves, however, this could be anywhere from 20 to 200 or so years. To put a bit of reference on it, Phinn's age is somewhere in the late 60s to early 70s. Rather young for an elf as she is already. And yes, despite the massive age difference and distinct eventuality that she will outlive him in the end, Phinn is still in love with Matt. Love makes you do wacky things.