Actually this raises a great question. What ages are considered MG and when does it go to YA? Putting aside content, and just addressing the age of the protagonist, I had one agent tell me that if the MC was 15 or older that no way it would work for a middle grade - she would reject it immediately because she couldn't place it as MG or YA. I am not sure I agree. Since my novel involves an MC that must battle an evil king, I thought putting him at 15 sounded more reasonable since you would expect some physical adeptness in order to participate in a sword fight. I am always told that kids read up so I thought 15 would be okay...the novel is definitely MG rather than YA. Any opinions on this would be appreciated!
Kids do read up, but they generally don't read that far up (obviously some do, but for the purposes of discussion and general guidelines).
A 15-year-old is in high school (I realize your protag may live in an alternate reality, but in this one...). Middle grade is generally around 8-12/13. A 10-year-old likely won't relate well to the situations a high schooler finds himself in, the language he uses (not profanity, just general discussion), etc. Their concerns are vastly different.
If it's an MG book, yeah, you have to age him down or have specific world-building reasons for why 15-year-olds are somehow equivalent to like 10-year-olds, which would seem to defeat the purpose in keeping him older.
It's a bit confusing, since different agents seem to have different ideas about the ages kids are in middle school. In our area, they're typically 6th-8th grades, so kids will usually be in the 11-14 range. High schools are 9th through 12th grade, so kids would be 14-18. But midgrade has traditionally been given as the 8 or 9-12 age range, with characters allowed to be a tad older, up to 13 maybe (since kids read up).
From these blog entries, it sounds like agents and publishers are getting away from a rigid focus on character age for defining the MG and YA split. I've even noticed some shelves at B&N that seem to be focused on SF and F for younger teens (shelved next to the YA fantasy), which I assume would be middle-school/junior high age. Really, it's about the plot and themes embraced by the book. But of course, it would be odd to have 14-year-old starring in a story with 7th grade concerns, or a 12-year-old starring in a story with 9th grade concerns.
http://writeforkids.org/2014/01/the-difference-between-middle-grade-young-adult/
http://kidlit.com/2010/11/20/is-it-mg-or-ya/
http://www.upstartcrowliterary.com/middle-grade-teen-unlocking-the-mystery/
Is there a reason why your novel with a 15-year-old protag wouldn't be a YA fantasy?
Even though 12/13 may technically be a MG protagonist age, it's also an age range you might be counseled to avoid by agents/editors. That's the grey area between MG and YA where it could be either, so it's safer to go with 11 or 14/15 and make sure you're clearly in one or the other.
15 is definitely too old for MG.
And if the only reason the character is that old is the whole swordfighting thing, well it shouldn't be an issue. It depends how realistic your book is (but there are always prodigies don't forget) but since you have an Evil King I assume it's fantasy and if that's the case you have a lot more flexibility to have exceptional heroes. Arya in Game of Throne is a kid when she learns sword fighting. I had a protagonist who was 10 and a half be a competent swordsperson too.
The issue is more voice, what's the voice of your work. If it reads MG then it should be MG. If it sounds more YA then it should be YA.
But to answer your question: absolutely 15 is too old for MG.
A lot of fighting styles don't need brute force, so it's not that unlikely that a child could learn them. But if it's something where they'd need to be a fair size, simply make them bigger. I've known ten-year-olds who are already adult height. An eleven-year-old who gets mistaken for fourteen is not that uncommon. Children vary a lot in how fast they grow.
I think 12 is a pretty standard age for protags in MG. I've always read that kids read up- so if you're aiming a MG book at 8-11 you want a 12 yr old so readers can read about an older character (if only slightly)
Just about all the MG I read stars a 12 yr old. 13 starts to get into "tween" fiction and I've read some of those too- but aren't books with 13-14 yr olds called upper MG now? But I agree that anything over 13-14 is too old for MG. That's a young YA protagonist.