This may not be the answer you want, but write the story first. Decide what it is afterward. It could be an older chapter book or young middle grade. The age of the protagonist and word count alone don't determine this. Reading level and the handling of themes come into play as well.
Not to steal anyone's thread, but out of curiosity...
I have a WIP that has a 10yr old protagonist. Using renlearn I've looked up the series I used to love as a kid, especially those most like what I'm writing. They ranged from 10k to 30k. Should I be shooting for 30k? And what age range would my readers likely be? I suppose that worries me the most. I don't want to dumb down my story, but also don't want it to be above their heads.
For further reference points:
My story deals with horses. The original Saddle Club series had older characters (about 12-13) and averaged around 30k. A more modern series, Canterwood Crest, also has characters older than mine with an average word count of 45k.