age difference in Kittens

I have a 18 week old male kitten and want to get him a brother or sister. I have a couple of questions.
Would i need to get a kitten closer to his age? or would it be ok to get a younger kitten?(there are some 6 week old kittens available for us)
Does gender matter when choosing a playmate for your existing cat?

In my opinion, I wouldn't get a six week old as a playmate for a 4 1/2 mo old. The older one may be a little too rough at playing and accidently hurt the younger one. If it were me, I'd look for one closer to his age...maybe 3 to 6 months old. And I think either sex would be fine at that age. How wonderful that you are looking for him a playmate!!

I wouldn't get a 6 week old, kittens need to stay with their momma's until 10-12 weeks. Unless of course they are orhpans and there is no momma, in that case I'm not sure when the recommeneded adoption age is.

Gender doesn't really matter, but you do need to get him neutered soon (if he's not done yet) and fix the new one as well.

6 weeks is far too young to be taken from mom/siblings. I'd look for one about the same age - minimum of 10-12 weeks old. You need an emotional and physically stable kitten. They need to stay with mom/siblings to learn important emotional behaviors.

M/F and M/M seem to work out better then F/F. But at this young age for both kittens if you like females better, get them now. Just be sure they are neutered by 4 months old if you are in the USA. In England, vets don't want to neuter before 6 months (you all need to push for earlier neutering/spaying)

I agree with everyone 6 weeks is too young. Before you get you new kitty read the Stickys at top of this forum Have a behavior problem - look here 1st, to help with introductions.