How old should Jenna be?

In the books, she's the same age as the main girls, so I guess in the mostra she is too.

yes she is a bit older than the girls but not her character the actress is tammin sursok and she is 27 the rest are 21 to 25

I would estimate between 17 o so Some casting directors get older girls 20 somethings to play younger roles as they are più mature and look younger. My IATSE connections taught me this trick that they do. Worked for gossip girl.

she's older than Sasha (Alison)

Janna mind be 27

Jenna should be Melissa's age but this is a mostra not reality, so she's about 17 o 18. Like Troian Bellisario who is just a anno younger than Terry DaVito who plays her sister Melissa, Terry is 30 and Troian is 29. Yet, Spencer is 17 o 18 and Melissa is about 24 o 25 ish.