young protagonisy as MC in mystery story

No. But a lot of people would. Write a good book. People will read it.

I would probably prefer an older teen, or someone in their twenties, but it would depend a lot on the story.

If it was a Nancy Drew or Hardy Boys type story, then no, I probably wouldn't be interested. If it was a more-or-less an adult story that happened to have a teen as the MC, then I might. That's just my own personal preference.

If I were you, I'd write the story I wanted to write, then worry about where it fit. If it turned out to be a YA story (or possibly even a MG), that might work as well as it being a more adult mystery. A lot of people have enjoyed Nancy Drew and those types of stories over the years.

They might. If it's good they will. I'm a big fan of mystery/suspense books, TV, and movies. A favorite of mine was Veronica Mars which has the MC as a teenager. I believe the series starts off with her at 16. And, though this wasn't quite a mystery, John Grisham's "The Client" has the MC as an 11 year-old kid and Grisham's writing is aimed at adults and I'm not a Grisham fan myself, but that was an excellent book. So it is possible. If that's what your story calls for, then write it.

I wouldn't read it unless the voice was quite grown-up. If it was written from the limited point-of-view of the seventeen-year-old, I would think it would be more of a YA mystery than an adult mystery.

On the other hand, many books have young protags but are written for adults. A fine example of this is "Extremely Loud and Incredibly Close" by J Safran Foer. It's not a mystery exactly but it's quite brilliant.

May I ask how old you are? I don't want to be nosy, but if you are close to the age of your 17 year old protag you might find it challenging (and unnatural) writing for an adult audience. If so, why not write a YA mystery? YA is a huge market, and you've got years to write adult-audience novels later.

Just my thoughts. Enjoy your writing

My protagonist is 15...but it is a YA Mystery. I agree, just write your story, and worry about who will read it later.

The protagonist in The Sweetness at the Bottom of the Pie is eleven and it isn't a children's book. It is, in fact, an excellent read. It just depends on whether your character can carry the story. So go for it!