It's never too late to learn a language but it's a bit of an uphill battle, and you're not going to learn it by just watching shows. You need to practice regularly and understand the grammar and sentence structure. You also have to speak it with other people to get feedback, you can't only learn to listen.
It's about brain plasticity and 25 is kind of the same as 45...
So they can learn French in an "anything is possible if you try" kind of way, but realistically unless they straight up move to France and completely dive into it, it's going to be a massive struggle to get to where they can even understand French shows without English subtitles.
Like, at a certain point people should realistically evaluate the amount of work and payoff they get from stuff.
Marrying a French person and wanting to learn their language? Yeah. That's probably worth the work.
Wanting to watch French TV without reading? Not so much
But that study was done on people aged 65+ for 11 weeks? I mean, sure, they didn't measure any significant changes to the brain, but that doesn't preclude changes forever. 11 weeks is not long to practice a language