ヨウ, は, よ, わ
leaf, plane, lobe, needle, blade, spear, counter for flat things, fragment, piece
JLPT3 G3 S12 F414
枼# is a depiction of three leaves growing from the top of a tree, suggesting slight objects (thin, flat leaves) piled on each other. 葉 adds 艸 grass/plant (for emphasis/clarification) → *leaf* → *thin object*. *Era* and *age* are extended meanings (← piling up of generations ← fallen leaves piled upon each other; compare 祖 and 昔).
[Full Kanji Etymology List]