#152.5 - As Chikorita age, the sweet aroma that wafts from the leaf on their head turns spicy, perking up surrounding Pokemon rather than calming them. Chikorita control this shift in aromas as they get older to encourage other Pokemon to battle with it.
#153.5 - As Bayleef mature, the leaf on the top of its head will dry out, and eventually fall off. The discarded leaf is often used by herbalists to create healing powder. A trainer can tell how old a Bayleef is by how open the petals around it’s neck are. Once they are completely open, the Pokemon is very close to its final stage.
Requested by anon, a-thousand—yearsss, and tr3eck0.