It’s not a problem at all. As you know NEET has no upper age limit no one is going to stop you considering that you are overaged. There is absolutely no hindrance for you to start the NEET UG Preparation. I personally know some people who joined the MBBS course at 22-23 years. You need to be dedicated and must go up with a rampant flourishment in confidence to crack the exams in the first attempt as a 20-year-old.