If you're changing careers or jobs after age 40, you may end up reporting to someone much younger. You're likely starting at a lower level in a new field, so the chance of working for younger people is higher. This often happens in industries like technology where people tend to be young and move up quickly to senior roles.
This is a difficult situation to handle emotionally when you're coming from an industry where you have a history and depth of knowledge. There are techniques you can use to develop strong working relationships and mitigate issues at the start and over time. The most important thing to remember is that age determines how long you've been working, but not how strong or knowledgeable an employee you are.