There's nothing "ageist" about saying two women are 15 years too old for their parts. I'm an adult, I can't play Annie. Nothing wrong with that.
Also, Disney said once (forgot where/when, sorry) that Anna is 18 and Elsa is 21. The ages of the princesses has always been known, even if it wasn't confirmed in the dialogue of the movie/musical (for example, Snow White is 14, Ariel is 16, etc.). Elsa is dark and brooding, so she could be a little older, but Anna is cute and naïve, which is something a 38-year-old will have trouble playing no matter what. Murin is a talent and she would've been perfect for this 10 years ago. To me, it's as if Disney didn't want to take any chance with fresh faces (or an interesting director, for that matter) and went the easy way out. Which they've done before (Sierra Boggess), so I don't know why this time seemed different.