Well, this question has a two fold answer. I was 14 when I started wearing my friends makeup (which I don't recommend- germs can come in different forms, even from your BFF, just saying!) while I was at school and would wash my face before my mom got home. I know, what a rebel, right?! But it wasn't until I was 15 that my mom allowed me to wear makeup, which consisted of face powder, mascara and lipstick. I've made a lot of mistakes through the years with application and color choices, but that's a whole other story that could be a blog post on it's own!!
The second common mistake is not correctly color matching your foundation. I can understand how it can be frustrating when you hear words like "undertone" and "pink, yellow, or neutral" which can leave you wondering if you should have brought a dictionary with you to the store! Check out my post on finding the right shade for you: Never Wear the Wrong Foundation Color Again! However there is no reason why ladies should walk around looking like an oompa loompa with too dark and too orangey of a foundation thinking they're looking tanned with the "J Lo glow" because trust me, that's not what it looks like! Stick to your color then add warmth with a bronzer (lightly!!) to achieve a more natural glow to the face. I'm not pointing fingers or making fun, but just do yourself a favor and don't fall into these makeup no-no's-think of it as my friendly makeup public service announcement ;)