Any dog can be trained in theory, and most in practice, regardless of age. Dogs LIKE to have their own "cave" - their own safe, secure place, so with most dogs, it's not too big of a struggle.

My brother's dog was not quite an adult, 9 months old, and she took to crate training quite well. Now, her "crate" has become the kitchen, with a baby gate separating it from the rest of the house.