Here's why:
* Noun clauses act as nouns. In this sentence, the entire clause functions as the object of the verb "believe".
* They are introduced by words like "that", "who", "what", "where", etc. In this case, it's introduced by "that".
* They contain a subject and a verb. The subject is "Stella" and the verb is "took".
Therefore, the noun clause is the entire phrase "that Stella took dance lessons at her age" because it acts as the object of the verb and contains a subject and verb.