How old is Jessica from Shakespeare's Merchant of Venice?

We are not told Jessica's age.  She is a young woman, old enough to marry and to be considered pretty by the young men in the play.  Considering the early age of marriage when the play was written, we may guess that she is about 18 years old.

She could be even younger.  It was not unheard of for girls to marry in their early teens.  However, she seems to be about the same age as Portia and Nerissa.  

Portia, though young, is clearly not a little girl.  She is old enough to pass for a young man when she dresses up as one.  In fact, she mentions to Nerissa that she will make her voice sound like a young man's when it's cracking as it changes.  Thus, she is passing for a boy in his mid- to (perhaps) late teens.  This means that she is probably a little older - say, late teens or very early twenties.  

Similarly, Jessica dresses as a page boy when she runs away with Lorenzo.  So she is probably not a girl in her early teens, but rather in her mid to late teenage years. 

Also, she spends 80 ducats in one day after she runs away from her father's house.  This argues that she is a teenager.