I would think that Simba was at oldest early 20s went he challenged Scar. He was still young and inexperienced. Which explains why Scar was hard to defeat.
Actually, the original planned ending was Simba being thrown off Pride Rock into the flaes and Scar being engulfed by blazes when they reached Pride Rock.
Scar was hanging off the cliff like Simba was, then Simba tried to help him but he threw him off like he did to Mufasa. But this was a deleted scene, and you could see Simba beng lit on fire.