Dear kiddo,
Have you ever noticed that there are more insects in a big park than in a small garden? That's because the age-area hypothesis says that the number of species you find in an area increases as the area gets bigger and gets older.
Think of it this way: when a new area is formed, there are only a few plants and animals there. As time goes on, more creatures move in, and they adapt to the environment. This creates more diversity, because different animals have different needs and abilities.
For example, let's say we have a big forest that's been around for hundreds of years. There might be deer, foxes, owls, and squirrels living there. But if we look at a small patch of trees that's only been around for a few years, we might only see a few birds and insects.
So basically, the age-area hypothesis says that bigger and older areas have more different kinds of creatures living there than smaller and younger areas. Isn't that cool?