It's not like she's always running around in cute attire, talking in a cute voice and such. I don't see how "acting your age" means you can't be childish every once in a while. Ayu seems like a fun loving person, I'd rather her be cute than looking like she has a stick up her butt all the time and like she doesn't enjoy her work. Ayu is just being Ayu
But I think the way she sings the end of Days live is annoying for other reasons. Control that vibrato girl >_<;