Since many twin pairs are born earlier than their Singleton friends, they can end up reaching their developmental milestones a bit slower than those single-born friends. They often speak later than single kids and may even start moving about a little slower than others.
How delayed they are can vary depending on the twins and their birth circumstances but it is typical to see twins about 6 months behind their single friends in speech development. They often have fewer words in use and they speak in more simplistic language and speak less often. Most twins will eventually catch up and grow out of this delay. And although we know that prematurity can cause this, it is not 100% clear why it happens. What is good to know though is that most twins catch up sooner than later.