At what age did you get married?
What age do you consider too young to be married or to even consider marriage?
& How did you know that your S/O was the one?
Personally, I'm engaged at 21 and I can't speak on what age is "too young". It varies from person to person and everything is not for everybody. Some people wait until their late 30's to even consider settling down while others marry young. 18 I think is the youngest I would ever consider marriage. It's also true that times are changing. In younger generations, many more people married young and that was socially acceptable.
I knew my SO was the one when after a particular incident (that would just end up being way too long to type and way too complicated so I'll keep it short) , he sat down and said that he accepts me for all of my faults, and all the bad things I had done in my life, and for hurting him prior to , and that none of that mattered anymore, because he would be there every step of the way. That for me was HUGE for him because he's not open with his feelings AT ALL and it meant a lot to me.