Based on the fact that he switches it on and off due to environmental context, this would not appear to be any sort of disability.
Forewarning: It has never been unusual to have groups of kids roller-skating or big-wheeling through our hallways and kitchen, and I might even strap on some skates myself, so consider the source here.
To me, the key question is how his perceived immaturity is having a negative effect, apart from just bothering you. When he's whooshing on the totter, is there anything unsafe going on? If not, and both are enjoying the activity, I wouldn't give it a second thought. Same with climbing under the table - who is being harmed? If Aunt Agnes is at the table in a short dress, causing spills, etc., then, not cool. Otherwise, I'd chalk it up to kids being kids.
It's not unusual to see a 9yo start to be anxious about transitioning from child to adult, and regress their behavior as a result. Gifted kids would be, I expect, more prone to this, and my DD started expressing this kind of anxiety as early as 5/6. It could also come from having lost something that they loved from earlier. This is why, for instance, I continued reading to my DD when she was 9. It reassured her that she didn't have to lose it just because she was getting older, and it basically got discontinued on her own time.
