I think he was just trying to get the point out that he
doesn't like Mufassa and has been under his rule for a long
timeand is tired of being left out. but then again like
everyone else has said he might have been talking about lion
years!!

???

Maybe they just put it in there because it sounded good.

Well, like Simbasguard and Outlaw, said, It may have been
talking about lion years...plus, it's a cartoon and the song
writers may have thought that that sounded good in the
song...

This question confuses me.

A decade is only 10 years and a lions average life span is
20, but it IS only a cartoon and not all of the song lyrics
have to make sence.

The Only thing I can think of is that Scar meant decades in
Lion Years not Human ones.