Any book that gets a thirteen year old to read is age appropriate in my opinion. I never read Colleen Hoover and I believe she targets a bit higher then age thirteen. Again, if your niece really wants to read them I would let her. But course it’s up to the parents. I personally believe that when someone isn’t ready to read something they’ll put it down themselves. It’s not like with tv/movies or games that kids get peer pressured to read things they aren’t comfortable reading.

Also you can never be to old to read something. I read adult, young adult and middle grade books. I even enjoy some of the picture books I read to my niece and my friends daughter.