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 o younger than Mufasa was.
