When is a good time to think about marriage?

Hi, I'm almost 21 and my SO is 22. We have been together for 19 months, and love each other very much. We know we aren't perfect and neither is or relationship, but we wouldn't change a thing. The topic of marriage and the future has come up a few times, and we enjoy the idea of being together for the rest of our lives. We really think we got it right with each other. He graduates this year and I'm still in college and I know this isn't the right time financially to be thinking about it, but I really love him and would say yes in a heart beat if he asked me to be his forever, officially. I don't know if he really knows this though. Any advice? Is it way to early to be thinking about this? I've thought about it for a few months and can't imagine being with anyone other than him, when ever I picture my future, he's in it.