As you are under age you will not be able to marry by registered marriage. But people get married in so many ways. Most common method, which we see in movies also, is to go to a temple and tie the mangalasutra or thali in the presence of a priest and one or two witnesses. Such marriage is not legally valid. But you will be able to go on unless the marriage is challenged by an interested party. As the girl's parents have objection for the marriage they can challenge it.
As both of you are above 18, you can live together with or without marriage. If her parents complain to the police, they cannot take any action against you, if so long as the girl maintains that she is willingly staying with you. Validity of the marriage is a different mater. In one year you will be 21 and you can go to a registrar's office and get married.
But there is one risk. People can change their mind, young people in particular. If at any time during the time you wait for your legal marriage, the girl can change her mind or you can change your mind. Then there can be problem.
So long as you do not live together, living together will be a great thing. But once you start living together, after some time, both of you, or still worse, one of you may get disillusioned and may like to quit. Then there will be heart break and such things sometimes even drive people to suicide. But after you get legally married, separation will not be that easy.
Have you ever thought why you want to marry this girl only? Are you not seeing other girls? The reason may be this girl may have made her intention clear to you, while you do not know about other girls. One year is a long period and any thing can happen.