We've got some discussion about first person versus third person, which I think is interesting, so let's string it along.
So, my debut novel is YA, and when I turned it into first person, it came alive. Now, however, I'm writing a middle grade, and when I tried first person it was flat as a board. My YA protagonist is 16; my MG protagonist is about to turn 13.
Now, they are also slightly different genres - the YA is historical/mystery; the MG is contemporary/realistic fantasy. And the voices are vastly different: in the YA, as historical, my protagonist speaks with more formal dialect (she's upper class), and in the MG that contemporary voice can come across as slangy.
The other issue may be character - in the YA, I had to really dig her out. In the MG, she's more alive for me right off the bat.
I'm interested to hear what folks think!