These days, they only do that at puberty. They seek each other out and form groups of five or six juveniles. They stay together until reaching adulthood, which takes a couple of years. The reason why they do this is still a mystery. Maybe they need to learn things from one another they can't learn from their mothers; or to get to know one another better before finding a territory of their own. No-one really knows exactly why.
But a strange change is taking place:
Two years ago, researchers reported finding a number of orangutans, again living in groups. Since then there have been more and more reports of the same thing. It is likely that the orangutans living in the small amount of remaining rainforest, cannot find enough to eat on their own and are helping one another to survive.
The 'man of the forest' is an important indicator of the state of the rainforest itself. If the orangutan is doing okay, we know all is well with the forest. This change is a very serious sign...