Why does there have to be a certain age to do what you want in life?

Why is that people put an age on having children and getting married? Why cant people have children and get married when they are good and ready? I mean it's there life not anyone elses so why do people need to wait till there like 35 and 40 to get married and have kids? I mean there are people who are glad that they done got married and had their child in their 20's and early 30's...