As we learn about Kovu's destiny, Zira states that HE was hand-chosen by Scar. Though many don't believe this to be possible as he should be much bigger than Kiara. But what if the harsh and deadly conditions of the Outlands, stunted his growth? This 'can' happen with lion cubs. Kovu
does have the features of a teen. E.g: Arm fur, mane tuft, cocky attitude. In fact, he looked similar to Simba as a teen. I suspect that he could've been 14 or 15 but Kiara would've been probably 12.
When I think of what that BRUTE did.. I get a little TENSE!