I think you have two questions. Maybe?
First is the question of age in middle grade. I think what makes middle grade "middle grade" is, at heart, the way it's approached/the tone and perspective, etc. But close to that is age. And most middle grade characters top out at age 12, maybe 13 for some. (The characters, I mean. The readers are younger, like 4th or 5th graders.) So to have a 14YO character feels like you're already in YA territory to me. 14YOs are freshmen in high school. Again, the tone of the story is an element, too. But the ages you are listing are going to be characters involved in much different experiences than middle grade.
Second is the question of aging up. I completely agree with you on the Penderwicks. I wanted to know what happened with the original characters. The little sister was cute, but I was really more invested in the others, and then the last story wasn't through their eyes at all. My personal, subjective feeling is that readers would rather follow a character they love as they age up than to switch characters with every book just to keep a book middle grade. I mean, imagine if JK Rowling decided that Harry at 15 was too old for MG, so now we were going to switch focus to Colin Creevey's little brother Dennis. Really, not so interesting! It does make things a little tricky for shelving purposes, but I just think that readers get invested in a character, and want to follow them.
Then again, if you just make it YA to start out with, then I think you don't have that problem.
I wonder if anyone else has dealt with characters moving from MG to YA here.