I was thirty and teaching a class of so-called 'remedial' children who had nothing to read that grabbed them. The stories I wrote for them weren't much good - but a publisher saw them and treated me as an author, and I responded by becoming one. About a year for a novel.
Shorter books vary.
All around me. One of the advantages of living a hectic life in one of the world's busiest cities is that there's no shortage of drama and excitement around me. I never use the people I know, but certain themes recur in relationships. In the creation of characters I use bits of various people. Hitting on the right name for a character is a great help, like the right costume is to an actor.