At a recent book reading, someone asked me “How did you manage to keep track of all the characters in your novel over many years? Was there any particular method you used?”. The short answer is meticulous notes on ages, events, and so on. Make sure you get it all right. You can’t have someone be aged 35 in a particular year and be shown as being 45 a few years earlier somewhere else in the novel !!
As they say “God is in the details”. I list complete details about each character. These are written sequentially starting with the year of birth. Then as the story moves on, I do a fair bit of cross- checking to make sure the facts tally. I made brief biograhies for each of the characters and that really helped!