I don't think it has as much to do with how old you are when you marry as it does with your mindset
Marriage is serious business, and unless you are ready to keep your promise of loving the person, and caring for them until you die, then don't do it. It's a selfless deal.
A woman i work with is a a wonderful relationship. She's been married over 25 years, have 3 kids, one of which is also happily married. her and Her Husband married at age 18
She says the key is to be selfless. She always thinks of ways to make her husband happy, and he loves her for it. he does the same for her
It's not the big things that count. It's the small things that really matter. She does something as simple as make his lunch and coffee for him for work, and he appreciates it. They have fun together, they are best friends. This is a true relationship in My opinion.
She says she married her best friend. not just someone you hang out with the most, but someone you can trust with your life, someone you share a bond with
Relationships are work, and i think that people who marry young often times don't realize this when they have to balance out so many other things such as getting a career,school, AND a relationship
But who says work can't be fun? Have fun with it!
I say marry only when you feel ready. Even if you love someone, only marry when you are ready for it.