The episode where Erin's parents would not give her permission to marry because she was 16 just aired. They wanted her to graduate high school first. This was episode 16 of season 5. Just two episodes later in episode 18, is when Erin graduates high school. I believe she is said to be 18 then. The writers should have gotten their stories straight, I don't think it was usually that inconsistent. :confused:

I was 17 when I graduated. Erin could have been as well, but yeah, I agree it was a little too soon. I wouldn’t be surprised if it was shot or aired out of order. This done in the aftermath of Ellen Corby’s stroke, and I imagine there was some rewriting of scripts and shifting things around.