Well that's a bit of a strange answer. She wonders if he just recently had a birthday? But that doesn't really matter, because that's a good enough response for her.

"Oh! Well, that's good to know! I'm 19, so you're not that much younger than me!"

Mitsuri gives him a bright smile, because she's delighted to know there's another younger kid she can make sure to take care of here. The sister instincts are strong in this one.