The first "big" things I did that could be considered Mustachian were buying an $18,500 house at the bottom of the market in early 2012 (at age 20), and buying my first car shortly after, my 53MPG 1988 Chevy Sprint for $1000, which I wish I still had.
Before that though, I was always careful with money, and didn't squander my money from summer jobs on stupid crap like so many of my high school/college classmates. I didn't have a cell phone until I was 18 (2009), and I paid for it myself, and it was a cheapie prepaid flip phone (though oddly, I'm currently on an even cheaper plan, on an even cheaper yet fancier smartphone).
My first entrepreneurial venture I can remember was early high school, when I discovered I could buy packs of Skittles in bulk and sell them at school for far less than the vending machine and still make a profit. The school didn't like that.
Going back farther, in middle school I was very much into video games, yet didn't want to spend much money on them. In total, I spent about $275 over the years on (used) Nintendo 64 games from about ages 10-16. I sold them all last year together on eBay for $295. Good games don't go down in value (plus I always hunted for deals on them). It also shut my mom up when I told her that. She always thought I was pissing away my money that I would never see again on video games. I could have been, but I was smarter than that. I mean sure, $275 of VTSAX would have been about over $500 last year, but I still think I made out pretty well while playing some fantastic video games.
My first larger DIY-related purchase was my first PC in 2008. I spent about $700 building it when a similarly-speced-but-overall-inferior prebuilt PC would have set me back at least $1200. I'm still using plenty of those PC parts today (with a total of maybe $250 in upgrades over the past six years).
Then I found MMM in August 2012 and went off-the-rails Mustachian for the most part.