A. In your case at present, if you get married, the girl's family can force the police or the law by itself can be a source of trouble. So, you have to wait at least till you complete 21 years.
B. Without knowing your background, financial support and education it would be difficult to be precise.
C. Respecting the bond of love between you both, I request you to sit and think realistically. Yes, it's a nice feeling to be loved and this is the age. But be brutally honest and answer the questions below
It is just infatuation or attraction or a platonic relationship taking a different turn?
Is the bond between you both strong enough to withstand the harsh realities of life?
What would be your options to support financially you both (once you reach the legal age)? Many boys and girls struggle to get a decent job even at 25-28 years of age.
What are the education and career plans for both of you?
Have you made a rough estimate of the cost of living and home expenses per month? Even employed middle-class couple struggles to make ends meet. What about rent, food, functions, travel, emergencies, social expenses and future expenses when the first child is born?
D. This is just a brief list, considering both your ages, you need to have support for education and then a job in hand. This would vital to starting your family life once you get married without the consent of the girl's parents.
E. So, have a few days of real discussion and let the heart not overrule the mind. Being positive that you both have a good bond of love and mature enough to wait for a couple of years, come to a sensible agreement that you both will focus on your education and be in touch regularly without visiting the girl's house, going out etc. I say this because, if your love is serious from both sides, this will convey the message that you two are mature enough to understand the needs of life and love.
F. After two years, once you complete your degree and get a job then meet the girl's parents and have the marriage proposal discussed once more. It would be better. After 2-3 years, you will be in a better position and mature enough to make a good choice.
G.If you both feel that yes, you don't think that you can face the harsh realities or do not want to see each other suffer, then part as friends.
H. Give the situation some thought and time and then decide.