[Summary of initial mail: The people in the books count their ages in years. These seem to have the same lengths as our years. But ours come from the seasons we have: The seasons in the seven kingdoms are much longer. How do they come to this way of counting time?]
Years are not based on seasons, even in the real world. They are based on how long it takes the earth to revolve around the sun... i.e., on astronomy, the position of the sun and moon and stars. Ancient monuments like Stonehenge and Newgrange served astronomical purposes as well as religious, and helped measure the passage of years, the summer and winter solstices, etc.