I think Patti and Janet probably stopped being the same age when Janet returned to the show in 1971 after being gone for a few years, though the age difference may have come earlier in the 60s. For one thing, Janet had children while Patti didn't, and when those children were SORASed in the 70s, Janet sort of got "bumped up" a generation, kind of like Ellen Stewart on ATWT. Patti left the canvas in 1975 and really didn't return as a major character until 1985 (I recall seeing Tina Sloan as Patti in 1975-76 but don't recall a single thing she did). If anything, Jacqueline Shultz was younger in 1985 than Tina Sloan was in 1976. In 1985, I suspect part of the reason for a younger Patti was to keep Jo from seeming quite so old.

DJ