I think they were pointing out the age, not the length if the relationship. a lot if people don't end up with the person they dated as a young teen because you can change so much from when you were 15 than when you are in your 20s.

You can become totally different so it's not unexpected to grow incompatible with someone you first dated as a teenager. obviously, some people are able to make it work which is commendable. but iys rare and I don't think they were thinking of hookup culture when they were 15 lol. people just change more in their teens