I often get the feeling that being under 30 is more important to Inc than other more obvious measures of success.
It likely has more to do with PR agencies than anything.
Well is this part of that 30 under 30 thing they do?
There's nothing wrong with focusing on a particular demographic.
Being "successful" isn't necessarily the point, and I don't think it needs to be. If the goal were to write about the most successful companies, there would probably never be a startup mentioned--and it would probably be an article you had no interest in reading. "Google, still huge. Exxon, yup they make a ton of money. Etc etc."