Hey Girl - good questions.
Maybe states differ, but from what I know, you have to be 18 or older to work at a counter, but no worries, because working at a counter isn't going to give you the experience you need to get into the industry anyway.
Getting started is unfortunately a long process, but building up your portfolio is
vital to booking jobs. The cheapest (actually free) and easiest way to do this is to sign up on modelmayhem.com. Make yourself a profile on there, and once it's approved go to the "castings" section and browse the castings for your city and state. This is the largest international modelling networking site, and there are shoots every day that need makeup artists. Even if you see a casting that doesn't call for a makeup artist, email them anyway and ask because most of the time, they need one. When you're doing makeup for the shoots that you set up, you'll be working "TF*", which means Trade For...which means you're trading your services in return for images for your portfolio.
do NOT focus on getting paid, focus on getting images from decent photographers. Also, as much fun as it seems, don't do more than 25% of your shoots with crazy loud or artistic makeup. Your paid work in the nearest future is going to come from brides, and brides (and news stations, and most other people in the industry that you may deal with in the later future) are going to want to see clean and natural work. Trust me.
Once you have a good amount of photos for your portfolio (a good amount means different looks, not the same model with the same look but just in a different picture) then you get yourself a cheap little website and business cards. Also register yourself on a few different wedding networking sites like weddingwire.com and weddingbook.com.
I know it seems like way more fun to get paid for doing photoshoots, but the fact is that photographers know they can get makeup artists for free, so unless you're an amazing makeup artist and the photographer has a highly paid project for himself, paid shoots are VERY VERY rare to come by.
Pursue brides! I WISH someone had told me that 5 years ago when I started. That's the best advice I can give you. Build your portfolio then pursue brides
Feel free to email me any other questions
take care
LC
*edit* - for the time being to get side jobs while still in school, just bug all your friends and people you know to let you do their makeup for their weddings