* She's a college student. This suggests she's in her late teens or early twenties.
* She's older than the main characters. Mordecai, Rigby, and their friends are all in their early twenties, implying that Margaret is a few years older.
* Her exact age is never stated. The show focuses more on their relationships and personalities than their specific ages.
So, while we don't know Margaret's exact age, we can safely assume she's somewhere between 20 and 25 years old.