I think his mane resembles that of a lion at least five years old by the time Simba is born (assuming Lion King time works like real time). The Lion Guard seems to indicate that Scar's plan to kill Mufasa was his second plan after his initial plan that led to him losing the roar. Both Scar and Mufasa are notably younger during the events as in the flashback. I'm not a lion expert, but their manes seem to point to them about... three? Maybe four? Anyway, since The Lion King seems to begin shortly after this, I'd wager Mufasa and Scar are both just around 5 years old at the start of the film, and Mufasa dies probably around what would have been his 6th birthday as Simba seems to be about 8 or so months at Mufasa's death.