"You must think about the future even if you're young."
Translation:Il faut penser à l'avenir même si tu es jeune.
Because this sentence is more of a general statement and not one being made to a specific "you" personally. You can tell because it says "even if you're young". If it said "even though you're young, then that would be talking about the person you are speaking to specifically. But this sentence is making a statement that pertains to all people who are young.
Also falloir (il faut) connotes necessity, while devoir connotes obligation or duty. So I usually end up translating "must" as il faut and "have to" as devoir. "Need" could also be il faut depending on the context. But devoir isn't really the right word choice for this sentence, so for instance on doit penser would also be incorrect.
But the preferred translation uses "même si tu es jeune," which implies that the speaker is addressing a specific individual and not just making a general statement. I got this wrong the first time because I answered "Tu dois penser à ton avenir, même si tu es jeune" because I pictured a father and son having a man-to-man talk and I came here to figure out why that was wrong. After reading your comment, when the exercise came up again, I answered "Il faut penser à l'avenir même si vous êtes jeunes," which was accepted, but Duo's "preferred" translation is "même si tu es jeune," which, to me, doesn't go along with a general statement.
In spite of the comments of linguaphile it appeared to me to be addressed to a specific person, telling them that they should think about the future even if they are young. It is purely a matter of interpretation I would probably use -even if - if I was addressing a specific person. On the other hand I might use though if I was addressing a group of people. If it is meant to be a general observation then surely the plural vous should be used.
It is not clear that it is addressed to people in general. So Vous devez penser à l'avenir même si vous êtes jeune should be accepted. there is nothing to say it is a general declaration. Il faut is general but that is not indicated in the sentence itself and since we are not mind readers we are not able to divine the future or the answer you are looking for. this is supposed to be a course in French not in mind reading.
I was struggling with when to use Penser à vs. Penser de; here is a good reference: https://www.lawlessfrench.com/grammar/penser-lesson/