I don't doubt that there would be kids that managed to get the game and log in at all, merely stating that Mfauli has a valid point that it would have an age restriction. That, the popularity and the new tech involved simply means there shouldn't be that many and as you said they should be well versed in gaming and not go all "I've never seen anything like that".

True Asuna is part of the poor writing but given the fact that we haven't seen any kids on the front-lines the only other thing we have to go on is Asunas statement. If there had been kids in the high level guilds she shouldn't be surprised since she's planned and seeks out the front liners when it's time to fight the boss. If there indeed are kids in the high level guilds it just reinforces the idea that it's badly written.