I adopted Monte at 6 years of age. I knew exactly his personality, unlike a kitten who will change. He was mellow, not hyper like a kitten, but still has a lot of play in him.

I adopted Y.Lee as an adult. We never knew his age. This is one thing about cats, you usually can not tell the difference between a 2 year old and a 6 year old. So why should it matter? I had Y.Lee for 8 years before his diabetes got the better of him. We never knew if he died at the age of 10 or the age of 14 or somewhere in between, but it did not matter. His life here was filled with joy. He was the most loving, huggable, laid-back, empathetic, mellow cat a person could ask for. And knowing his age would not have changed that.

Calvin was also about 5 years old (a guess) when we adopted him. And the majority of the others were around 1 year of age.

Kittens are fun, no doubt about it, but adults are more stable. Somewhat less work to keep them occupied and out of trouble. If a certain personality trait is important to you, then an adult is the only way to go. And even though it seems impossible, I believe that the adults are more grateful when you show them love that they might not have received in the past.