Learn Russian: Хотя он молодой, у него есть опыт.

No, хотя (although) is very straightforward! Just like in English, you place it at the start of the clause. It doesn't force you to change the word order of the rest of the sentence.

Why is there an н at the beginning of него? Shouldn't "him" be его?

You are right that the normal genitive form of он is его. However, when third-person pronouns follow a preposition like у, Russian adds an н to the beginning to make them easier to pronounce. That is why it becomes у него.

Why does the sentence use у него есть for "he has"?

This is a great time for a quick refresher! Russian rarely uses a direct verb for "to have." Instead, you say "by someone there is something." The formula is у + genitive case (него) + есть (is/exists) + nominative case (опыт).

Why is there no word for "is" in он молодой?

In the present tense, Russian almost always drops the verb "to be." You just put the subject (он) right next to the adjective (молодой) and the word "is" is naturally understood.

Is the comma before strictly necessary?