Learn Spanish (Spain): Muchos jóvenes protestan porque el alquiler es muy caro. - Many young people are protesting because the rent is very expensive.

When you make joven plural, you add -es, which adds an extra syllable. To keep the spoken stress on the same vowel (the 'o'), you have to add a written accent mark: jóvenes.

Why does the sentence use the simple present protestan instead of están protestando for 'are protesting'?

In Spanish, the simple present tense is frequently used for actions happening right now or around this time. While you could say están protestando to emphasize that they are actively in the streets protesting at this exact second, protestan is the most natural choice for a general, ongoing situation.

Can jóvenes be used as both an adjective and a noun?

Yes! Here it acts as a noun meaning 'young people'. It can also be used as an adjective, as in chicas jóvenes (young girls).

Rent prices change, so why use es (from ser) instead of está (from estar) for 'is very expensive'?

Using es muy caro describes the rent's general characteristic—it is just an expensive thing overall in this context. If the speaker wanted to emphasize that the rent is unusually high right now compared to what it normally is, they could use está muy caro.