I agree with whatswhat, I have seen quite a few people who are in their 40s and not married. Some got marrited but no kids.
There is no fixed rule that we have to get married and have kids. Sometime I fell like people don't even know what they are doing, they are just doing what others are doing - thats why our population is 7 billion while we were 3 billion in 1960. This is little bit out of topic but I do really believe that there are lot of people out there that should not have had kids or got married.