The short answer is that you have to be 18 years or older.
You must be 18 years old to buy bitcoin, as it is a legal requirement in most countries. The exception to this rule is if you are buying bitcoin from a family member or friend, then the age limit does not apply.
In the United States, for example, you have to be 18 years old to buy bitcoin. In China, you have to be at least 18 years old and have an ID card or a passport with your name and photo on it before you can buy bitcoin. There is no universal authority that oversees the bitcoin market, unlike in traditional currencies where governments control the supply.
In order to buy bitcoin, you need to create a bitcoin wallet and then purchase the currency from a bitcoin exchange. You can also buy bitcoins from friends or people in your community, but this is not recommended because it will take up too much of your time and there is no guarantee that you will get the same value for your money.