0-5

Stiles is five years younger than Derek. They grow up together, pining after one another, but never admitting to it.

Derek is a werewolf, which means he has a soulmate. It's common knowledge now, but it used to be well hidden.

At first their parents think maybe it's a fluke; Derek is just excited about the new addition to the family. As they grow older they grow certain that it is a lot more than that.

They watch their children awkwardly hide their infatuation for each other as they grow up and stumble through life together.

Or

5 year age gap. Protective Derek. Clingy friends. Friends to lovers. Soulmates. Nobody dies. Hale and Stilinski families are super close and cute. Fluffy and eventually smutty.