Ryuutaro closed his fan as he replied.
"I'll be fifteen in a while. Though I can't really tell when because we traveled through a few dates. I'm just going with my actual birthday."
Which reminded him that he didn't even know which date they currently had in this world. It could be basically anything, with the way their time-travel-magic worked.
And when they got back to Hayato's actual time, it could be another again for all he knew.
Odd enough that didn't even startle Ryuutaro anymore.