Minecraft wiki entry for Kelp, associated text: Kelp, when planted, is generated with a randomly chosen age value, which can be checked when pressing F3‌[Java Edition only]. The age value of a newly planted kelp plant varies randomly from 0 to 24. Each time the kelp grows in height by one block, the newly generated top of the kelp plant increases its age by 1. When the top block of the kelp plant reaches an age of 25, it stops growing. This means that kelp can naturally grow to a height between 2 (if the first kelp plant had an age of 24) and 26 blocks (if the first kelp plant had an age of 0).