They always use older actors who look young to play teenagers. That has been going on since forever. Half the "teenagers" in the film are closer to 30 than they are teens.
Teens tend to not be very good actors and can't bring depth of emotion. I feel like the teen actors on Game of Thrones are quite terrible. Not everyone is a Chloe Grace Mortez or Dakota Fanning.

She looks right for the role. I thought she was about 22.

Wow I had no idea. She looks really young. I thought she was around 24-25, same age as Dylan. In regards to the story, it doesn't really matter in my opinion. As long as she looks young, she can play younger characters.
Whose the wiser?
-How many times do you have to make the same mistake?
-Till I get it right.

I thought she was early 20's, I thought she could be a new young Angelina Jolie seriously. Never saw on screen before. She probably has a baby face to keep her very young looking.

Also they don't have the same energy for action scenes (and Brenda has has a really good amount of action)
LOL, spoken like someone likely under 24. I agree she is pushing in comparison to the age of the character she plays, but her lack of energy is not the issue. In fact, all the best stunt performers in Hollywood are between the age of 30-45. Your knees don't suddenly give out past the age of 25.

I just saw her in the movie. She did extremely well. I could not tell she was 30 years old. She looked to be no older than 25 in the movie.

Girl, calm the beep down, Wikipedia set her age like 2 months ago and this post is old. I already know she's 23 and TST stills show her and I know she's gonna be amazing and I already saw her in Insurgent and interviews and I love her.

"Also they don't have the same energy for action scenes"
Sseriously? Even if she is 29, she's not elderly! Lots of stunt doubles are in their early 30s and 40s too. Tom Cruise did his own stunts for M.I. up until his 40s. Age has nothing to do with it.
Besides, if Rosa has a lot of action scenes in the film, she would have gone through months of extensive physical training with personal trainers to keep her fit, which they would require actors to do AT ANY AGE. It doesn't matter if she's 29 or 24 or whatever, she looks and fits the role and that's what matters most.

But sometimes actors of that age usually quit because of their age . example: Crystal Reed
Also they don't have the same energy for action scenes (and Brenda has has a really good amount of action) some say that she's 24 and others say that she's 29. However I hope she fits Brenda. I'm really looking for her performance due that I've never seen her work( I haven't found the movies and series where she has been)
Calm your passive- agressive tone you basic bitch.

"she looks the same age as Kaya and Dylan."
Thats why she got the part,Since the directors don't know her age they don't care as long as she looks the part
You want tah fack wit me? You ah fACKING chioah boi compahed tu me ah chioah boi