""Isn't she a little...mature to be playing this role again?"
I suppose that that depends on how old Norma was at the peak of her career. If she was 25 in 1910 and had a career until roughly 1931 she would be roughly 46 at the end of her career. If we agree that the musical takes place in 1950, which is the year that the film version came out then she would be 65. Glenn Close is currently 68, so I would say no, she is not necessarily too mature for the role. It seems perfectly reasonable."
The time line is fairly well-set in the text of the musical. It does indeed take place over 1949-1950, as the party-goers at Artie's New Year's Eve party sing "1950 will be my start." We know Norma was very young when she started her career, as Max, her first director, sings "When we met she was a child, barely sixteen." Joe also tells her "Nothing's wrong with being fifty, unless you're acting twenty." Since Norma was meant to be a contemporary of the great silent film stars of the 20s it is safe to assume that she was born right around 1900.
So Glenn Close may be too old to still play the role on film, but on stage, especially a large house like the London Coliseum (around 2,500 seats) that won't matter as much. It's also safe to assume she'll be wearing some fairly heavy, gothic makeup, similar to what she wore on Broadway, which will also help to blend some of those years away.
"Now if Judy Khun could reprise the role of Betty Schaeffer...I highly doubt it...but it'd be heaven."
Now that would be stretching beyond a mere suspension of disbelief. Judy is extremely talented, but hearing a woman who is nearing 60 tell the leading man that she is 22 would be just comical. And, quite frankly, I've never really cared for her on the recording. She never seems to capture the innocence and freshness that I always got from Meredith Braun on the London recording.