Research.

"Three, it's rude to refer to Doctor like that." The one chastising the young Valkyrie this time was Shizuru, not Nine.

"Well, he is old and a man so…" the young Valkyrie remarked.

"I'm not that old, you know." Sam said with a sigh.

"Does that mean I'm an old woman?" Shizuru asked, pointing a finger at herself. "I'm only a year younger than him." 

"Eh!? Seriously!?" Zero Three stumbled backwards, looking utterly shocked.

"Yes."

"Why are you so shocked?" Sam asked.

"Of course I would. I thought you were at least ten years older than her." The young Valkyrie spoke with a serious expression. "Maybe you'll look better once you start exercising more."

"I… guess you have a point…"

Sam already plans on getting back into shape so her remarks didn't bother him that much.

"You should do a lot of swimming." Zero Three said, getting excited as she once again sang the praises of her favorite sport.