Another reason for later development may be because your child is focusing on some other aspect of her development, for example physical growth and motor exploration. Occasionally, late talking may indicate physical problems such as deafness, or neurological disorders such as autism, but in this case it's likely to go with other noticeable signs of abnormal development. A tiny group of late talkers may also lag behind in cognitive development generally - these tend to be children who have trouble understanding the language of other people. Most often though, late language development is nothing to worry about. Most children who talk later usually catch up later, and learning to speak early does not mean your child will be especially bright or good at reading.
Genetic variations can account for later language acquisition.
It would seem that part of the machinery for learning language is inbuilt into the brain and can be inherited, and some research indicates this may have more to do with matters such as the speed of learning new words, and the ability to understand others language. Brain maturity also plays a part, and of course this too varies very much from child to child. For example, as Broca's area develops, the region in the brain for language production, children develop speech and grammar. Amazingly, the brain knows which areas the baby needs to be ready first. Wernicke's area, the centre of language comprehension, develops a good six months before Broca's area even starts.