Definitely a young kitten is better.
Or even better, two young kittens.
I had two brothers (several years old) one got run over and killed, within two weeks I got two young kittens. I didn't want the one who was left to get used to being on his own – that's why I acted so quickly. I got two kittens because I know how much kittens like to play, just one kitten would have pestered the older cat but with two, they had each other.
At first the two kittens were very aggressive towards the older cat which worried me as they were tiny compared to him. The vet said not to worry as he would not harm them on account that they were so small. Sure enough, although he would occasionally give them a slap, it was never in a nasty way and with the claws retracted. Within a few days he was grooming them, and after a week it was as if they had always been together – they're all inseparable now.
An older cat will be seen as a threat, and cause all sorts of problems – and the fights will be nasty. First impressions are very important, and if the experience is very unpleasant, it could have lasting implications. So do the first introduction gradually – start with scent, which is all important, before a visual introduction. Don't worry if young kittens seem very aggressive – a large cat can be frightening for them until they realise that it won't harm them. Good luck.