Hm, can you give an example of a situation your character gets into that requires her to be 16? I'm wondering if there would be ways to work around that obstacle, depending on the type of situations you mean, in order to age the character down to MG range.
Also, depending on the type of book you're writing, you can have a bit of flexibility in MG when it comes to character ages. Fairy tale retellings are one example -- think Ella Enchanted, which is solidly MG, but Ella and the prince get married, so she's clearly older than the typical MG character.
The closer you are to a modern setting, I think, the more the character generally needs to fall into a certain age range. My contemp MG fantasy protagonist was 13, but was asked to adjust age to 12. No big deal there, but 16 to 12-13 is a much bigger change, so I definitely see where you could be running into issues.
Looking into Hi-Lo is a good suggestion. You might also take a trip to your local library and ask for some recommendations for younger YAs. It might give you a sense of how to change your book to a YA if that's the route you ultimately decide to take.