plenkreska

Take it apart and it explains itself: plena plus kreski plus -a, "having grown fully". English says "full-grown" with the same two ideas in the same order, which makes this one of the compounds a learner can decode on first sight and then never forget. The main root goes last, as always in Esperanto compounding, so what the word is really about is the growing.

The noun is plenkreskulo, an adult person, with -ul- doing its usual work of turning a quality into the person who has it. Plenkreskuloj nur is the sign on a door that English writes as "adults only". Beside it sits plenaĝa, "of full age", which is the legal word: plenaĝa means you have reached the age of majority and may sign contracts, while plenkreska means your body has finished growing. A tall fifteen-year-old is arguably plenkreska and definitely not plenaĝa.

It applies to animals and plants as readily as to people, which is where English "adult" starts to sound clinical and Esperanto does not. Plenkreska arbo is a mature tree, and nobody blinks.