Yeah, a career in makeup definatley has a sort of ladder-effect. You have to start out at the lowest of the low, take baby steps, and prove yourself at every point before you can go anywhere awesome. Oh, and I don't mean to say your mom is wrong, but you can seriously do makeup your whole working life (well, until you're 65 and your hands are shaky.) There's a certain percentage (like 89% or something) of people in the world that don't enjoy their work. Makeup artists don't fit into that statistic
I too, someday aspire to be with MAC. However, when I walk into a MAC Store or counter, I see women mostly around 25 or older that are working there. Most have come a long way to get where they're at, and that's why they're so respected and confident. What I'm saying is I don't want to rush myself, and you shouldn't either. By the time I am with MAC I want to be confident that I'm close to being the best I can be. I want to have an outstanding portfolio, impressive resume, and be exactly what they want: a GREAT makeup artist. As you're working for other cosmetics companies, always keep MAC as the light at the end of your tunnel.
The way I'm doing it is starting out with smaller lines, and gaining sales experience and technique. After I feel like I'm good, I'll work with photographers to get TFCD/TFP shoots to start building my portfolio to show. After working with two or three lines or stores for about a year each, I'll probably feel like I'm ready to take on MAC, and sky's the limit when you're with them! You'll get paid more and be more in demand for wedding makeup, get to do fashion shows through the company, and whoever you want to get hired with after that will most likely hire you. MAC has the best training in the industry and is the epitome of perfection to people who seek your skills.
My advice is just to take it slow, be patient with transfers and promotions, and always ALWAYS do your absolute best from the beginning of your makeup career to the end.