When kelp is broken off by a piston and the base kelp is left to continue to grow, the age of the kelp no longer increases by 1 with each growth.  The age displayed in the debug screen seems to be random with each growth after being broken by a piston.  To recreate this, place a piece of kelp down and notice how the age goes up by 1 with each growth. Then use a piston to break the kelp off, but leaving the bottom kelp to regrow.  Take note of the new age of the kelp, and how it no longer increases by 1 with each new growth.  It seems to be randomly generating a new age.