Her response comes too fast. The kind of response you have when you aren't really thinking about it. She just wanted to do something 'cool' and wasn't thinking about the time. She probably didn't even comprehend years in her young mind.
"Do you really?" He kept moving, speeding his pace. He's showing off now. Showing what the human body can do when you keep it in peak physical condition for over seventy years and keep training it that whole time. It's the one reason he can keep up with people who should by all accounts be stronger and faster. They are to a degree, but he's just better.