How do you know that you are ready to get married at age 16 or 18?

I can tell you right now you are not ready. IF you have to ask then you are not.... You still have so much ahead of you, that to tie yourself down to that big of a committment would be silly. If you and this other person want to do the right thing, then maintain the relationship you have now, plan for a future together and both of you should finish high school and go to college and get out in the world and experience life. You can still do this together and you will find if you are meant to be together forever through all of the trials and tribulations it takes to keep yourselfs on the right path now.....