The life expectancy issue in Middle Ages is based on many things, but basically age 30 is an average. You have to take into account the high infant mortality rate, war and death in childbirth, which massively reduces the average age of a society. Plenty of people in Medieval times lived to age 60-70 once those variables were removed from the equation. I would imagine that a witch/wizard would live at least to this age and probably longer as they wouldn't have to work as hard (manually) as a Muggle.
Wikipedia says that life expectancy in Medieval Britain was about thirty years. So basically, a Muggle back then would be born, married at age eleven or twelve, perhaps slightly younger, push out as many children as they could, and then died.