We better get ourselves a clear definition of "adult." 
Let's not state it's because of age, because I'm technically an adult if we're going by that but I sure don't act like one. XD
<Snipped quote by DearTrickster>
Anyone older than thirty who holds a respectable position or long held job, or is successful. Two of which King Arthur is a good candidate for.
You can be above 30, hold a respectable position or long held job, and/or be successful yet still not be an adult because you fail to act like it. :P
I divide being adult up into three areas:
Physically. Emotionally. Mentally. I'm pretty no one is an 'adult' in all three of these, lol. *looks at some of the behavior of the heroes currently and chuckles*