Not to be a contrarian, but:
Katharine Hepburn: born 1907
Adam's Rib, 1949 (age 42)
African Queen, 1951 (44)
Pat and Mike, 1952 (45)
Summertime, 1955 (48)
Guess Who's Coming To Dinner, 1967 (60)
The Lion In Winter, 1968 (61)
Rooster Cogburn, 1975 (age 68)
On Golden Pond, 1981 (age 74)
And these are all starring roles or very…