The relative clause in this sentence is "that was beyond his young years." To identify a relative clause, you can look for a subordinate clause that starts with a relative pronoun (such as "that," "which," or "who") or a relative adverb (such as "where" or "when"). In this case, the relative pronoun "that" introduces the relative clause. The relative clause adds more information about "intelligence," providing a description of its quality or characteristic.