I'm an MB. If my nanny came to me and said that she was planning to go back to school and also that she's learned that she really enjoys young children the most, so therefore would be looking for a position that is a better fit for her at this stage, I would understand.
I might be upset to be losing her, and to have to face finding someone new, and maybe I would worry about whether there was a deeper reason and she was just being polite, but I can totally understand a nanny preferring a certain age range and I don't think there's anything wrong with that. You don't have to get into the behavior of their specific kids at all that way so hopefully it won't be difficult.
Good luck.