Does the German translation for the noun "Adult" end in "e" or in "er"? Both endings are shown in various online dictionaries, such as dict.cc or leo.dict.org.
The ending depends on the gender and on the definiteness (if you use it definite or indefinite):
A female adult = Eine Erwachsene
A male adult = Ein Erwachsener
The female adult = Die Erwachsene
The male adult = Der Erwachsene
(And of course it also depends on the grammatical case: »Ich sehe den Erwachsenen.«)
This is because this word is a nominalized adjective, and although it is a noun it inherits some properties from its adjectival origin, and so its inflection also depends on definiteness.