They cast a wll over 50-years old woman as her voice actor so I daresay she wasn't supodes to be very young even at the beginning, not to mention after Simba grew up. A lion's old age starts when they turn 10-years old and females can't have cubs before they turn 4 years old. Adult Simba was at least 5-6 years old.
I daresay at the end of the film Sarabi was supposed to be around 10 years old, perhaps even older. Depending on whether she was supposed to be the same age or younger than Mufasa was.