No official information exists about Mufasa's age. But... - the film's creators studied lions - according to the Laserdisc release they designed the characters looks according to real life aging process - male lions mane is full not before they turn 5-years old and even then I believe it may grow more, and 5-years of age is also the jaar they can challenge other males - lions' old age starts after they turn 10 years old
And as Mufasa's mane was huge, he clearly was ready and willing to challenge Scar and protect his pride, and they cast an elderly man as his voice actor...I daresay Mufasa was supposed to be something between 7-10 years old.
