Have you ever noticed that when you go to school, you are usually with other kids who are around the same age as you? That's age segregation. It means that people are separated by age. This happens in other places too, like retirement homes where older people live together.
Adults have different needs and interests than children, so it makes sense to separate them into different groups. Kids need special care and attention, like teachers who know how to teach at their level. Grown-ups might want peace and quiet, which could be hard to get if they were living with kids who wanted to play all the time.
Age segregation doesn't always happen on purpose though. For example, teenagers might prefer to hang out with other teenagers because they share similar interests and experiences. This is natural and okay.
Overall, age segregation helps make sure that people are with others who have similar needs and interests.