Okay first you DO realize you are calling Carl, Coral right?
Seriously though, I do sense some off screen politicking in this. They seemed to indicate that he was on the show for more years to come and then this....
Was college going to interfere with filming?
Is the studio releasing him so that he can live his life? Noble, but somehow I sense that isn't the case.
Did the kid do something off camera that is perhaps best left a mystery?
Was the kid and his father looking for a bit too much money?
Or now that the kid is turning 18 perhaps labor laws, union rules, etc etc were going to kick in and start causing more interference?
What I am confident of is this: hope the kid invested wisely and pursues a good education at college because right now he is so typecast from the Walking Dead that if he isn't careful he WILL go the way of other childhood actors that were exploited by Hollywood and their parents who grew up and couldn't find anymore acting gigs.
Remember the show Different Strokes? Looks like the Willis actor has turned things around for himself, can't say the same about Dana Plato and Gary Coleman
Jodie Sweetin and the Olsen Twins from Full House, all still alive but could be considered damaged goods from their issues.
Anissa Jones from Family Affair, overdosed at age 18
Jonathan Brandis from Seaquest, another suicide issue
Sawyer Sweeten from Everybody Loves Raymond
And more examples but I think the above are enough.
The Hollywood industry has absolutely no qualms at all about destroying or eating its young.