IMO all and everything should be killable, kids and advlts alike. However, killing kids should have severe repercussions in communities, making them hostile towards you. Word should also spread, which would make 90% of all cities hostile against you eventually. This would go along with the whole role playing thing if you want to be a murdering [censored].
I think all publishers dislike the idea of killable children in games. This would be very negative PR because 99% of media is always such a bunch of sensitive hypocrites. Most people understand that things portrayed in games are fictional and that we don't carry the actions over to real life. Almost all action games involved killing or destruction, but we don't go out and kill and destroy because of it. Equally we wouldn't go out and murder children all the sudden. I mainly think it's just a very silly PR thing. Just look at the media coverage on GTA5 around strip clubs. They were not concerned about all the murdering, but they had problem with [censored] and touching the strippers. Imagine the hilarious [censored]storm if you could kill kids.
Short answer; Legally a person is an advlt at 18 in most countries, but how the world works is that age doesn't seem to add to either ethics or morals, but looks does. This means that if a character looks young enough to be a kid, they most likely will be considered a kid and hence be 'unkillable'.