Lizzy was a 6 month old kitten that was born in a barn when we got her. She had never been inside or seen a litterbox, but she has never had any problems using it. She is a year old now, and still is faithful to it.
So it is my opinion that barn kittens are very easily trained to use them.
Also many many years ago, when I was first married to my second husband ( long since divorced now) we had a barn kitten given to us. He also took to it quite easily. He was much younger and we just kept putting him into it until he finally used it, after that it was never an issue either.