Song learning in birds usually has a sensitive period. A young white-crowned sparrow, for example, must hear the right model early or it will never sing normally, and its adult song crystallises and stays fixed. This age-limited pattern is the rule across many songbird families.
Mockingbirds belong to the smaller set of open-ended learners. They copy the songs and calls of other bird species, and also non-avian sounds including frogs, alarms and machinery, then deliver each copy in a run of repetitions before moving to the next. Repertoires reported for individual males run to dozens of distinct song types, and they continue to grow across seasons rather than freezing after the first year.
Two things are commonly overstated. The imitations are convincing to human ears but are not perfect acoustic replicas, and the fine structure often differs from the model. And the unmated males that sing through the night are the ones drawing attention; a paired territorial male is far quieter after dark.