Learn German: Sind alle Wartenden hier Erwachsene? - Are all the people waiting here adults?

It is a nominalized present participle. First, you take the infinitive verb warten (to wait) and add -d to make the adjective wartend (waiting). Then, you capitalize it to turn it into a noun for a person: der Wartende (the waiting person). Since we are talking about multiple people here, it is plural.

Why does Wartenden end in -en here?

Because nominalized participles still behave exactly like adjectives when it comes to endings! The word alle (all) triggers the weak adjective declension, just like the definite article die does in the plural. Therefore, the adjective takes the -en ending, giving us alle Wartenden.

Why does Erwachsene end in -e instead of -en?

Erwachsene is a nominalized adjective (from erwachsen, meaning grown or adult), so it follows the same ending rules as Wartenden. However, in this sentence, there is no article before it (it is just adults, not the adults). Without an article, it takes the strong nominative plural ending -e. If it had an article, it would be die Erwachsenen.

Could I say alle wartenden Leute instead of alle Wartenden?