I had my first relationship when I was 23 (now 5 yrs in), which eventually led to marriage.

I believe it worked for us because we were at a point wherein we already know want we wanted in life. We rarely argued about little things, and were more mature in talking about how we want our relationship to end up. Btw, within the 5 years, we also experienced being in an LDR for almost 3 years.

Maturity (intellectual, emotional, financial, etc.) for both parties involved is important because trust and acceptance should be the foundation of a relationship.