Martha made the mental translation of 'Theta = Doctor' and came up against a blank wall with 'Otherstide'. "Okay, so I'm a bit older than you."
Not that she was going to tell her age. She wasn't ashamed, but after spending all that time with the Doctor and Jack and being the youngest by centuries, it was really weird to find a Time Lord younger than her. "We have birthdays, yeah, though a lot less than your sort will."