I second the 6mos or under kitten. My elderly cat passed away when my littler one was about a bit over a year or so old. He seemed rather lonely so we decided a companion right away might be for the best. He accepted the kitten completely even without the careful intro - in fact it was impossible to keep them apart even though we tried. Neither of them care for other cats if they see them outside, but they are pretty good friends. They sleep together and everything.