Taylor Momsen is 23 years old right now but when she got her start on Gossip Girl she was about 13 (turned 14 shortly after). I believe she was playing a freshman in high school so 14 years would have been the right age if they did not put her in adult situations with actual adults.

To give a bit of context.
Actor
Character
Current Age of Actor

Blake Lively
Serena van der Woodsen
29

Leighton Meester
Blair Waldorf
30

Penn Badgley
Dan Humphrey
29

Chace Crawford
Nate Archibald
31

Ed Westwick
Chuck Bass
29

Jessica Szohr
Vanessa Abrams
31

Taylor Momsen
Jenny Humphrey
23

In this scene she is most like 15 going on 16. Her costar was probably 25:
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In this scene she dancing around half naked which is whatever. But notice that the character Nate finds her. The actor is 8 years older then her and they kept trying put them together.

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