When asked what math is, my inner mathematician wants to come up with some rigid and precise definition for it. However, thinking about the question a tad more carefully, I realize that perhaps there isn't one. I could say math is a way of explaining what happens around us in a logical and numerical way. While sure this is a fine definition of math, there is also so much more to math than just numbers and logic. New discoveries in mathematics are occurring all the time to describe anything and everything about the world, and with these the definition of math is growing as well. So for me, the best way I could define math is by likening it to an infinite series, how mathy of me. Just like with the next term in the series, each new discovery broadens the scope of mathematics and as a result the definition becomes that much different than before.
Perhaps you could see from my definition of math that I am a big fan of models and examples so, I think math milestones would somewhat resemble how we learn math as kids. First you start recognizing numbers and amounts, like hey four apples is more than 2. At this point I still didn't fully grasp concepts such as addition or subtraction but that did come next. After that, it was most likely multiplication and then division. Meanwhile, also at about this time, or maybe a little before, I'd say kids would start learning about simple geometry like shapes at this time. Geometry is sort of weird for me as I'd say its easier to learn than algebra, the beginnings of it at least, however its very possible geometry didn't become a thing until after multiplication was discovered. Finally you go into algebra and calculus and everything else.