The difference between relationships between kids, young adults, and grown-ups lies in the ability of the folks involved to resolve conflict with conversation.
Kids tend to resolve conflict with tears, tantrum, and manipulation. Young adults tend to avoid conflict where possible and prefer to avoid face to face conversations (hence break ups over text). It takes grown-ups to sit across each other, listen, and reason.
The nature of any given relationship isn’t a function of age – age and maturity aren’t correlated. It is instead defined by the maturity of the parties involved.
As a result, a very small percentage of relationships between adults fall into the “grown-up” category.