"I can't seem to find Cusack's age online, but I don't think she's all that much older than Jessie Mueller (who is 33). And even if she is significantly older, I don't think she reads that way. She played a teenager for half of Bright Star."
Cusack has yet to divulge her age to the public (even in her NY Times profile, she didn't give it to the reporter, for reasons pertaining to the fact that she played 16 and 38 in Bright Star). However, having read an article about her a while back, I recall that it said she moved to England at the age of 20, and this was after having worked on the QE2 in 1992. So, suffice to say she is 43-44 years old which, yes, is too old to play Jenna. However, I wholly agree that due to her versatility as an actress, she could still totally make it work and would be sensational-- she did, after all, believably play 16 in Bright Star to moving and hilarious effect.