What is the function of when she is a girl of twenty in a sentence

The phrase "when she is a girl of twenty" functions as an adverbial clause or phrase in a sentence. It provides information about the specific time or condition that is being described. Here's an example sentence:

"He plans to propose to her when she is a girl of twenty."

The phrase "when she is a girl of twenty" functions as an adverbial clause of time in a sentence. It provides additional information about the timing or condition under which an action is taking place. The sentence would need to be provided to give a more specific analysis of how this clause functions within the sentence.