I agree all children are different - my dd is two years younger than ds1 but has the same level of vocabulary thereabouts. My ds2 however, is 21m and can barely say anything - he can say mum, dad and sheep and that's it!! I was worried at first but he is well-developed in other ways, he can do the same jigsaws as his older brother, who's nearly 5 for example.
The three of them are all so different, and advanced or behind in different ways, and they all play great together and with other children, and enjoy the company of adults too. So I'm happy, because it doesn't matter what age they learned to speak in sentences, or to differentiate all their colours, or learn to write their name etc
However, children are children. They aren't adults, and it's our job to a.) let them enjoy their childhood innocently, b.) encourage them to develop skills needed for adulthood and life, e.g. manners, right from wrong, social skills, empathy etc and importantly, that they are not the centre of the universe!!
I know that sounds a bit harsh, because they are the centre of their own and our universes whilst they're young, but if they grow up thinking this they will turn into selfish adults lacking basic skills.
It's important to remember that children are children for a reason, and it worries me that some people think their children are their property! They're not there to be best friends with their parents (as a young child) or part of the couple that are the parents (ever!) and to treat them this way is bordering on emotional abuse. Sorry, not meant to sound harsh.