I first realised that I wanted to be a mathematician when I grew up when I was 3 (not even kidding). I used to stare at the pages in my Dad's book 'Engineering Mathematics' by K.A.Stroud, absolutely enchanted by the beauty of the notation.
I would ask 'What do all the 'd's' mean Dad? How is it letters and numbers at the same time?'. 'You'll find out when you're a little older' came the reply. I now know about 70% of what's in that exact copy of the book, and I'm not stopping there
So, share with me the moment when you realised that you wanted to be a mathematician