I don't see the point in speculation on something that is already a done deal as far as casting goes (Whether or not the movie is made is another story). James Mangold wants to use Dafne Keen, Mr. Craig Kyle (Laura's creator) who just posted a page or two ago on this very site is writing the script for the character with Dafne in mind. Hugh Jackman and Patrick Stewart have also given Dafne their blessing.
I'm curious to hear a solid explanation as to why you feel we "need" a grown up Laura at all. You can refer to Ambaryerno's previous post as to why Dafne remains the perfect choice, age-wise. Something flimsy like trying to chase Box Office never works out well for the majority of these comic book movies. The ones that make bank are the ones that try to take risks with thoughtful storytelling and interesting characters that also respect the source material.
Just look at the DCEU, WB set out with the singular, narrow-minded, tunnel vision goal of making bank at the Box Office instead of simply putting out good movies with good storytelling and growing their brand. And they did make bank....for a time. Now look at what happened with Justice League. Sony tried this too and STILL IS. Sadly they will continue to learn the hard way.