The age of consent varies from one country to another. Unless someone can correct me and tell me otherwise, the age of consent in India is 18. If you have sex with someone who is under the age of 18, you are committing a crime.
This assumes that you are living in India, but if you are not, then you need to do a quick check on the Internet about the age of consent in your country. For example, in the UK, the age of consent is 16. In Spain it is 14, in France and Germany is 15. Some countries don't have an official age of consent, such as Japan. In some cases, it is left to judges to decide whether a crime has been committed, according to the physical and mental maturity of those concerned.
That is what the law says about the age of consent, and as I say, you would be best advised to check on your facts, and don't rely on people like me to be accurate, as I am liable to make mistakes like anyone else.Now let's look at the circumstances around your particular situation. Your 16-year-old girlfriend wants to have sex, And is encouraging you into this. You consider that you are not ready for this, as it is your first time. We all have a first time, and with some people it is quite worrying occasion, as we are all worried about making it enjoyable, both for herself and our partner.
It is a simple fact of life that girls mature before boys in most cases. It is common enough to see a girl going out with an older guy, because a boy of her own age is probably not as mentally mature as she is. This would appear to be somewhat your situation. She is eager for sex, and you are reticent, probably because of your fear of not being experienced. How do you think you are going to get experienced until you take the plunge?
One thing that I find worrying, and that is the fact that you have given no thought to contraception. Or at least, let me say this, you haven't mentioned it in writing to us. If you and your girlfriend are going to have sex, then you really must wear a condom. Do not expect her to take some morning after I pill, as these things have some unpleasant side-effects for a lot of girls. It's not fair to expect her to be taking all the precautions.
We on this website cannot be seen to condone underage sex, so while I can't tell you what you should or shouldn't do, make sure you are aware of the law, and the possible penalties if you get found out. Having said that, literally millions of young guys and girls have sex when they are below the age of consent. In Western Europe and North America, underage sex is normal. Girls getting pregnant at 13 or 14 years of age is commonplace enough. I remember when I was at school, in East London, there were several girls walking around with a big bump in front of them, as they had been made pregnant while still at school. Girls would regularly disappear, after the rest of us watching their tummies get bigger and bigger, then they would return sometime later, with no bump, but bigger boobs, because a busy breastfeeding their new babies. It's commonplace enough, anyway you look in Europe or North America, and who knows where else? The thing is, if she is under the age of consent, you could get prosecuted.
Even if you were not prosecuted, if you and your girlfriend were discovered to be having sex, it would cause a seismic evil in both your family and hers. I wouldn't want to be on the receiving end of angry parents, particularly the angry parents of a 16-year-old!!! They would quite rightly point out that you, at 21 years old, are supposed to be a responsible adult, and they would point the finger at you, calling you a paedophile, regardless of whether that label is really appropriate (which I don't believe it is). So if you are going to do this, for God's sake be sensible, and be very discreet, both are your safety and hers. You have to remember that the law takes no account of your feelings for each other. The judge would not be the least bit interested in the fact that she is obviously emotionally and physically mature enough to want to have sex. She may love you, and you may love her, and therefore sex is a natural consequence of that love, but when you are stood before a judge, trying to explain all this, don't be surprised if he turns a deaf ear, and finds you guilty. What you have to do is to make sure that you aren't caught in the first place!!!
I hope this helps you to make some sensible choices.