There is no firm answer to this. It doesn't help that you don't indicate what you mean by "too young." I was 21 when I married my husband (19). It's been 10 years, and not always easy -- but always worth it. I will admit we married too young, and if I were to advise my younger self, I would advise waiting a few months -- or, at the very least, discuss parenting/ desired number of children BEFORE we married.
As far as age and relationships go -- on the one hand, there are sociological and relationship studies that indicate those who marry later are subjecting themselves to more difficulty. Something to do with being more set in our habits, more independent, and less willing to make the necessary changes and compromise necessary in any relationship.
On the other hand, marrying too young can create a sort of imagined parallel life -- where every time you have a fight, you wonder, "What if? What if I'd waited? What if I'd given him a chance to meet other people? Where would I be now? What would my life be like?" And sometimes those questions make life unbearable, the unanswerable, unknowable dream that life could have been different and better.
You may want to read a few hubs I've written on relationships, some in response to questions similar to yours.
1. To Marry or Not to Marry: http://hubpages.com/relationships/to-marry-or-not-
2. The 10 Commandments of Modern Relationships: http://hubpages.com/relationships/The-10-Commandme... (despite the title, not religiously-inclined. Promise.)
3. Are Soulmates Real?: http://hubpages.com/religion-philosophy/Are-soulma