Of course, Sherlock Holmes has been portrayed as a man of many ages, from a stately gentleman to a Robert Downey, Jr.-aged 40something, to a, well, young Sherlock Holmes, so it's not hard to imagine that this might work, but Miss Marple seems much more of a piece with her age and social situation: a bit of an old loner who's never been married, which I think we can agree is not to be Jennifer Garner's lot in life.
You can't make a film nowadays without a romance angle, so I'm betting that Disney's going to throw Ben Affleck or Will Ferrell in the mix, or something. Regardless, it's a bizarre casting choice. I like Garner more than most seem to; she can play up emotional vulnerability pretty well when she wants to. That's not quite a Marple-ish quality, though...unless Disney wants it to be, I guess.