Colleen Hoover's books are considered new adult romance books, which are generally considered books written for college-age audiences. Books by Colleen Hoover have mature content that may be explicit and darker in nature, which is why they're usually recommended for an audience over 18 years old.
That said, if your teen is interested in Colleen Hoover books and you're okay with them reading the content, then they can read them too. Some of her books may be appropriate for ages 14 and up.
As always, it's up to the parent to decide what their teens should be reading, but it's a good idea to check the classification of any of the Colleen Hoover book series and stand-alone books before purchasing.