Is Boomer out of kitten stage?

As most of you know, I am the paranoid sort. Recently Boomer had a bladder infection, but is now doing well with only two or three clumpings a day instead of eight. However, I am now worried about his activity level. Before Boomer was a raging ball of fur. Running, playing, jumping, activity was constant all day. One must understand that this cat would hardly sleep during the day. However, now he's more of the snoozing sort. I understand that cats usually sleep 16-18 hours a day. He is generally playful in the morning and right before bed, but I guess I'm just worried because he sleeps all day where before it was common for Boomer to run from window to window all day. Is my little cat growing up? Is he out of his kitten stage? Or should I be worried? Any ideas would be great. Thanks!