I previously reported this problem but used a modified version of Minecraft so it was resolved as inaccurate, I've recreated the problem in vanilla Minecraft 1.13.2:
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. As you can see in the attachments, the kelp that is one block tall has an age of 15, and after it grew, its new age was 13. The kelp should continue to age by one after breaking a piece off with a piston.
Linked issues
duplicates 1
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Resolving as duplicate of the more general MC-145485.
NO! Please don't!
You will destroy all kelp farms!
If a kelp (below of the piston) age by one - after about 15 piston "cuttings" the age will be zero and it will stop growing. End of farming!
Do it the other way round please!
Every kelp below the cutting piston should have an age maxed out (15?). And if this seems too high please set it fixed to 7 or 8. That is no game-breaker and is also a bit "like in real world" - plants regrow when cutted. Normally plant don't stop to grow after been cut.
Hope i described the problem good enough. Thanks for reading.