Learn Spanish (Spain): Según una encuesta reciente, los jóvenes leen menos. - According to a recent survey, young people read less.

It is because of Spanish pronunciation rules. Words ending in "n", "s", or a vowel naturally stress the second-to-last syllable. In the singular joven (JO-ven), the stress naturally falls on the "o", so no written accent is needed. When you add -es to make it plural (jóvenes), you add an extra syllable. To keep the spoken stress on the "o" (which is now the third-to-last syllable), an accent mark is required. You will also see this happen with words like examen and exámenes.

Why is "young people" translated as just los jóvenes instead of using a word for "people"?

In Spanish, you can often turn an adjective directly into a noun just by putting an article (el, la, los, las) in front of it. While you could perfectly well say la gente joven or las personas jóvenes, simply saying los jóvenes is the most common and natural way to refer to young people as a group.

The sentence uses según with a noun phrase (una encuesta reciente). If I want to say "according to me", do I use según mí?

Surprisingly, no! Most prepositions in Spanish are followed by prepositional pronouns (like para mí or de ti), but según is a special exception. It is always followed by subject pronouns. So, "according to me" is según yo, and "according to you" is según tú.