What age do you have to be to become a Christian?

There is no set or minimum age at all. Some churches teach that you must reach an "age of accountability", and usually make this about 13. The reasoning is: because that was the traditional age for Jewish boys to undergo the "Bar Mitzvah" ceremony in which they assumed full religious obligations, Jesus Christ was a Jewish boy, Christ died for our sins, therefore that applies to Christians . There is nothing in the Bible about having to reach a particular age before you can become a Christian. The only basic prequisite is to repent and believe the Gospel:-

Mar 1:15 and saying, The time is fulfilled, and the kingdom of God draws near. Repent, and believe the gospel.

You can become a Christian at any age if you understand what this means. It is not an age of accountability, but rather a state of accountability.