I was a late bloomer–I really didn’t get into makeup until I went off to college, so I had just turned eighteen! I think it’s important to balance having fun and maintaining a young person’s self-image and self-esteem. You don’t want makeup to become a crutch or give them the idea that makeup is required to be beautiful, but you don’t necessarily want to forbid them at all! I would say subtle things like tinted lip balm, sheer gloss are great ways to start out when you’re young.
MAC made me fall in love with makeup — it was MAC pigments (which could have very well been fake MAC pigments, actually, given I bought them off of eBay), and it was pretty much a downward spiral from there.
For the most part, I organize by brand and then by product, but for brands that I might have only one or two items of, then I just sort by product type. I just like the way everything stores/fits when it’s sorted within a brand, since those products will fit well together.