Here's what we know:
* She is a young woman. She is described as "fair" and "witty," and she has a playful, even flirtatious, personality. This suggests she is not yet old enough to be considered a spinster or a "serious" woman.
* She is old enough to be romantically involved. She is in love with Orlando, and she is willing to disguise herself as a man to follow him into the Forest of Arden. This implies she is old enough to be considered marriageable.
Therefore, Rosalind's age is likely somewhere between 16 and 20 years old.
It's important to note that age was a more fluid concept in Shakespeare's time, and there wasn't the same emphasis on strict age ranges as we see today.