Growing up, there are many milestones. First, we turn 16 and can finally drive; then we're 18 and we can vote, go out to clubs, etc. "adult things." Then, finally, after 21 years we can drink a legal beverage. Sure, most drink before then, but once you are 21 and no longer have to wash x's off your hands, can get in for free most places, and get drunk... after getting to the club, it's a freeing feeling.
Going out with friends is always a good time, even if you're over 21 and your friends aren't. But, there is something different about being able to walk up to the bar with your friends. There is a greater joy in ordering two drinks and getting to hand your friend the other no questions asked. When you drink around underage friends, the bouncers watch you. The bartenders question if the drink is for you, or a friend who can't legally be drinking. Rightfully so, if caught the business will be in trouble. And, regardless of the ethical debate of if the drinking age should be 18, the fact is if you aren't 21, you have to pregame, meaning you're probably going to be shitfaced before we even leave the house, and your buzz will fade much quicker than mine when I get to continue to drink for the rest of the night.
So, enjoy your night in, or go out with other friends! I promise your older friends still love you and will go out with you another time. Once you turn 21, and all of those friends get to take you out, they will know all the best drinks to order at the bar, and probably buy you a few rounds of drinks or shots! Being the youngest might have its own struggles, but coming last has many advantages!