Culms that come up in a particular year will do all of their growing that year. They will not get taller, or bigger in diameter, or add more branches, or become bushier in subsequent years. If you are seeing "older" culms doing this, it means they are culms that came up this year. Bamboo culms have some annual turnover in their leaves, and will re-foliate if they got stressed, dropped leaves, and then recovered, or if they are in a severe climate that damages the foliage in the winter, but doesn't kill the culms.

My bamboo do a change-over of leaves but no actual branching or lengthening of old culms. There's a sudden drop of leaves which carpet the ground but the new leaf regrowth is almost simultaneous so you don't see much change in the canopy. Except for one thin, very tall species I have that does look spindly for a few weeks.

The whip shoots come from damaged new culms. I get lots when I break off emerging culms to control the bamboo. It doesn't happen with all the species but with one they just go ballistic in all directions.