I could careless about Jen's age. Age in Hollywood doesn't matter. There are actresses in their 30s who look young enough to play Katniss.
My only with Jen as Katniss(Movie-Katniss) is her height and weight. Book-Katniss is a 100% underdog. She's not that attractive, she's one of the smallest tributes, she's boring and she's not the smartest. Movie-Katniss, on the other hand, is a front runner. The only thing against her is that she hails from the poorest district.
Movie!Katniss is no smarter or more "fun" than Book!Katniss. And I wouldn't be so sure how attractive Katniss is or is not. She doesn't consider herself attractive, but she wouldn't. Haymitch says upon seeing her and Peeta that they are "attractive enough" to look really good when the prep teams finish with them. Her prep team are exasperated with her for her lack of proper Capitol grooming, not because she's naturally unattractive. And when they do dress her up and give her a makeover, she's obviously attractive enough to capture the imagination of the entire Capitol. And the Capitol people are mostly shallow - would they really be going gaga over an unattractive girl? True, they don't talk about her looks all the time, but there are a few references such as Haymitch mentioning Thread spoiling her "pretty face" that's supposed to be on the cameras. She's certainly considered one of the "young and attractive Victors", and she herself mentions that in the parade, she and Peeta look "young and strong and beautiful" like the Victors are supposed to be. The only times someone says she's not attractive is Finnick teasing/making a joke in Catching Fire (when Katniss asks him if this is the first time he's not pretty and he retorts with a question how she managed it all her life) but that's a joke; and when hijacked! Peeta saying that she's "not very pretty", but the non-hijacked Peeta called her "beautiful" in Catching Fire. Finally, the fact that Capitol people wanted to give her breast implants, which is often taken as proof that she's built as a stick, doesn't actually prove that, since there's no mention of anyone wanting to give her butt or hip implants, and many women with normal or curvy build have average sized or smaller breasts and would probably be asked to get breast implants to be the nation's poster girl, if the Capitol's tastes are anything like the current Hollywood tastes where really big and firm breasts are the standard to aspire to, which leaves 90% of women "lacking".