In the past, there were families with a dozen or so children. However, ın the modern world, having an only child became more common by the increase of tendency to have promising career and with the help of medicine. It is clear to see that having or being an only child effects parents and child. From the child's perspective, being an only child has both pros and cons.
Generally, in the families with more than one child, kids envy each other in terms of having attention of parents and it sometimes ends up with fighting. As an only child, they have no siblings to compete for attract parents' attention. They automatically have both their parents' full attention. It is well known that the more children, the more burden to family. It means, families having an only child can provide a better life to their child with holidays, good schools, more possession, opportunity to study abroad and so on. Unlike the others, those who are only child spend most of their time with adults. One of the effects of spending a lot of time with adults is that they learn early on how to think like adults. It exactly what makes them more mature than the others.
When it comes to disadvantages of being an only child, the "Little Emperor Syndrome" comes to mind. "Little Emperor Syndrome" refers to the Chinese situation including families and their single child. It is a case in which parents lavish their attention, love, resources on this one child of their, and as a result, the child becomes spoilt and behaves like "Little Emperor". In old age, when parents need someone's help to survive, an only child needs to carry parents' responsibility alone as they have no siblings to share responsibility. For example, my grandfather cannot even take a shower on his own, he needs my father's help to take a shower. One of the most important disadvantages of being only child is they are not able to have nephews or nieces. Also their children have no uncles, aunts, cousins and the like.
To cut a long story short, families with an only child have become more common compared to past. For the children, being an only child has both advantages and disadvantages. Even if some disadvantages can be coped with the help of parents, the others are unable to be avoided.