If they qualify as a dependent of someone else, even if that other person doesn't claim them, they cannot claim themselves.

If you look at the federal information worksheet it asks....CAN someone claim you? DID someone claim you?

According to the comics - YES

In real life, probably not - Part time work, living at home, eating mom's food, sleeping in bedroom paid for by dad. I can't imagine that they are supporting themselves. 

No.

"I cannot find a definitive answer..."

Definitely "NO"