Yes.
The age depends on the country you live in, what type of sex you are talking about, and how old your partner is.
The United Kingdom is 16 for everyone. Some countries make a difference between sexual acts such as anal or oral. In the United States some states even have "age disparity" rules. Most Islamic states require both partners to be married to each other. Under US law US citizens when abroad to be 18, regardless of any local laws.