Answers (3)
Neither or both.
The 20 year old WOMAN could be into all sorts of self defence, and could beat a man twice her size, or be a lazy couch potato on her way to an early heart attack because she's out of shape and eats like crap. Or, she just doesn't know how to defend herself, but is otherwise in somewhat decent shape.
The elderly man could be frail and unable to help himself in terms of strength, or he could be one of those guys who's been fit his whole life and hasn't let age get in the way. It's all about how you treat your body.