Usually there is no mechanical ventilation system in older buildings. Residents are dependent on natural ventilation through vents in facades, windows or drains. By opening these vents and windows it’s possible more or less to regulate the natural ventilation according to your needs. This is the healthiest method because the fresh air is supplied directly in to the living areas.
On the other hand with natural ventilation a lot of heat is lost, which can lead to unnecessarily high heating costs. You should be airing regularly, but don’t open the windows in winter for longer than half an hour.