You are just 18 years, it is just infatuation or a platonic friendship that is changing towards love or do both families know each other for a long time and you want to keep the family friendship going on.
1. You are not yet of legal age as per the Indian legal system of marriage.
2. Age difference is not a legal issue but a social issue. There are many examples of succesful and unsuccessful marriages with an age difference of around 5 years.
3. General men in India at the middle class level are not keen on marriage an older girl. If your bonding is strong then you would weather it well.
4. Getting married at a young age raises concerns in the minds of people who are married because, the responsibilities and commitment a marriage needs is not easy to put in words. It is honoring the needs of your spouse, the arrival of a child, the commitment to have money, place and time to make the marriage work. Generally, without sounding harsh, boys at the age of 18 -25 are still teenagers as far as marriage is considered. Think of all the duties of a married man and then take a decision (once you are 21 years old).
5.Just as a simple practical exercise, see how much of money is needed just for the kitchen and food at your home, the rentals, the vehicles, the things you need for a decent life and work it out for 2 people. Then if you are comfortable, you can check on the cost of getting an admission for LKG in a good school. It is not easy, unless you are blessed with lots of family money.
6.If you have siblings, a wedding in the family at a very young age raises eyebrow and people start asking questions
7. I would say, if both of you love each other, take the next three years as the test of time. You complete your graduation, get focused on a job and learn means of earning a living.
8. If by the end of three years and once you cross 21 years, you both still have the same intentions and can honor each other as 'Equal' partners in the marriage, then you can go ahead with the blessings of both families.