When the movie was out, Farrah’s part at the “New You” shop was all of five minutes long, and with bad acting.
That didn’t stop the local movie marquee from putting her name on the movie marquee instead of Agutter.
When I see it as an adult, the gaping plot holes are big enough to drive a maze car through. Oddly, the movie was better than the book (having the whole thing end at 21 was kind of ridiculous, especially since nursery and especially cathedral weren’t all that great.) Thirty made a lot more sense.
Plus, there weren't a whole lot of teen actors at the time. Anyone they hired would've looked quite a bit older. (And teen nudity would've still been an issue until the 80s, when it was all over the place.)