The noun clause in this sentence is Stella took dance lessons at her age.
A noun clause is a dependent clause always started by a noun, and can work as a noun itslef.
For instance: it is possible to say
1. "No one could believe that Stella took dance lessons at her age" or
2. "No one could believe THAT/IT/SO/THE STORY"
- Note in the second example we replace the complete sentence for a noun.