How can one respond to those who say that Islam is younger than Christianity?

The religion of Islam is the religion of all prophets, from Adam to Muhammad, peace be upon him. Allah has chosen it for His servants, and it is based on submission to Allah and dedicating worship solely to Him. While the Sharia of Muhammad, peace be upon him, abrogated many previous rulings, the foundation of the religion, which is the Oneness of Allah (Tawhid), remained constant in all messages.