Ignore what the others say, if you like her then just tell her. You must be really young, age gap doesn't really take into account when you get older. I've been dating someone eight years younger than me and my husband is 3 years younger than me. It really doesn't matter, it's who you are with that counts.
Age really is not a big issue and you should just follow your heart. My one piece of advice to you is that you are still very young, may be get to know her a little bit more. Be friends and see what life take you, there is no rush to be boyfriend/ girlfriend right now. You have plenty of time, wait until you both are more mature so it may be easier for her to accept you.