As soon as it needs to take place.
However, you don’t want to wait too long. You want to introduce the incident and the character’s motive early on so that the reader has a reason to keep reading. You can show your protagonist’s life in just a few pages and make it effective, but I wouldn’t go much farther than that without including a motivation or at least hints of the inciting incident.
You can still write parts of your protagonist’s “perfect” life after the incident happens through flashbacks or just mentions of things that happened in the past. It can be a part of your character’s growth and development.
Write what you have planned and see how it works out. You may need to begin the story in another place, but not always. If you can pull it off, the incident can be held off for a little while longer. But your beginning would have to be really good. Another exception for holding it off would be fantasy and sci-fi worlds where world building obviously takes up more time and space than in other genres.
