I quite liked having a slightly older Sarah in the 1976 revival. Ernestine Jackson was 33, and James Randolph as Sky seems to have been around 50 or 51. Can't find a date of birth for him but he was said to have been 88 when he died in 2013. It was poignant, especially to see a Sky who had been living this way for so long. As for Jackson, she was really great in the role. I remember thinking that her Sarah was a 30-year-old virgin, and that gave it something in addition to her having been terrific in the role. Randolph was not as good as Jackson, he seemed a singer more than an actor, but he looked great and his age added something.
Mignini was around 44-45 when she was Sarah. There is no date of birth for her online, but it says in some places that she was Miss Teenage America in 1965. I found that it it was actually 1964 when she won, and she was said to have been 17 at the time. If they had cast a Sky who was her age, perhaps it might have worked, but Peter Gallagher was 36. We know that, unfair though it is, it has generally been considered more believable by audiences when the man is older than the woman rather than vice versa. Indeed, that was the case in 1976.