To me he didn't seem to age much over the course of this series. How old was he supposed to have been when Bryton McClure started playing him in year 2? I guess about 3? Which would mean that he was supposed to be about 11 when it ended?

Also, by the final season or two McClure wasn't even billed as a regular and yet the kid who played 3J was which was odd. 3J is even in the last few shows and Richie was nowhere to be found.

Yeah Richie was the character that kept appearing and disappearing throughout the series. In the last few seasons he was hardly seen and was just a "special guest star" when he appeared. A good assumption can be when he was not seen was that maybe he went to visit his mother Rachel.

Yeah, that was the only season when they finally mentioned that Rachel was no longer with them.