Learn Finnish: Aikuisena minä juon paljon kahvia. - As an adult, I drink a lot of coffee.

Aikuisena is in the essive case, which adds the -na or -nä ending to express "as a [something]". To build it, remember that Finnish words ending in -nen (like aikuinen) change that ending to -se- before adding most case endings. This gives us the stem aikuise- plus -na.

Is being an adult considered a temporary state in Finnish?

Yes! Even though adulthood lasts a long time, phases of life (like being a baby, a student, or an adult) are treated as roles or temporary states. That is exactly why we use the essive case here to mean "during my time as an adult".

Why is the word for coffee (kahvi) written as kahvia here?

It is in the partitive case because of the word paljon (a lot). In Finnish, paljon always forces the noun that follows it into the partitive case. Because kahvi is a basic word ending in -i, you simply add an -a to the end.

Could I put aikuisena at the end of the sentence instead?

Yes, you could say Minä juon paljon kahvia aikuisena. Finnish word order is quite flexible. However, starting the sentence with aikuisena is very natural because it immediately sets the context or timeframe for the rest of your sentence.