DeAndre Kane did not get drafted because of his age. He was a better player than Marcus Smart that year. Age is 100% a factor in the draft. Kris Murray will be in his Age 23 season next year when he's drafted.. Why it is rare for players that are older to get drafted is that most of the players declare for the draft when they are younger. So just be elimination there are only going to be a few NBA type players that stick around in college for an extra year or two that are NBA players. It has nothing to do with "aging out". They are just not good enough.
If for example, all the projected NBA draft picks for this coming year and the next 4 years decided that they were just going to stay in college for their complete eligibility they wouldn't "age out" they would still be top draft picks. It is just that now they all go early. Also, I don't recall seeing a 23 or 24 year old player in the draft