I have had my cat for 10 years from being a kitten and she has NEVER sat on my knee, unless you count the time she had anaesthetic and sat on my knee for about a minute.
BUT, for some reason today, whilst she was sat at the side of me, I decided to pick her up, and she instantly started padding, then, she actually laid down and went to sleep on my knee for 1/2 hour, whilst she was laid on my knee I hardly dare move or breathe, though I don't know whether it is a bad thing, with her age, or a good thing that she finally decided to reward 10 years of love and attention with a cuddle, i just hope she decides to give me more cuddles.