I wonder if you could give your take on the difference between MG voice and YA voice? I recently had an agent comment that my voice was sometimes firmly MG and sometimes more YA, which confused me!
Hm... it is hard to say what that agent might have meant without looking at pages and hearing the comment in context. There are two different things that they might have meant, though:
TONE
Though the person said voice, they may have really meant tone. I am going to make some sweeping generalizations here. GENERALLY, middle grade books have a sweeter, more child-like tone than YA. That is NOT to say that they can't be a bit snarky (like, Roald Dahl or Lemony Snicket) - but children are very rarely physically endangered by a real thing (witches curse? yes. historical fire? maybe. school bus shooting? no.) There is nothing more than holding hands and maybe a kiss between boys and girls, if that. There is no objectionable language.
The stories often feature kids trying to figure out their place within their circle of friends or family.
YA, on the other hand, CAN be sweet, but it can also have rapists, murderers, molesters, fairy drug addicts, regular drug addicts, sex-addicts, cutters, bingers, purgers, cross-dressers... need I go on? If your book contains the darkness of the human soul, the passion of the human heart, the lustfulness of the human groin, the cursing of the human mouth, or any "content" combo thereof, people will say it "feels YA".
The stories often feature teens trying to figure out their place in the wide world and/or going through some major first time stuff - romance, heartbreak, friend dying, etc.
VOICE
To me, voice is what makes a great character THEM. This is a totally individual, unique thing. Junie B. Jones sounds different than Charlie Bucket who sounds nothing like Sabriel who is night and day from Fancy Nancy. It could be that they felt that your character did not sound like a 14 year old -- either too young or too old, or just not authentic.
The kind of problem with having a 14 year old protag is, they ARE so very in-between. Is it young YA or is it upper MG? Kinda depends on what you are going to put your character through.
Best of luck!