1962 for me, which was the season before my senior year in HS. Switched my allegiance from the Pirates to the Colt 45s, and here I am. We are in the Golden Age, and long may it last.
My Dad & I played a homemade dice baseball game when I was a kid ... taught myself long division keeping stats, which in retrospect may have been my Dad's motivation all along. It did get me an extra recess period in 3rd grade during math class, but I digress. We each "managed" 4 teams in the National League (playing a 42-game round-robin schedule), so when expansion hit I got the Colt 45's (somehow) while he got the Mets. That could not have been geographical, because it was the summer we moved from Michigan to South Carolina. As a side note, an expansion team typically does a lot better in a league that is 100% based on luck. :-) This was also one of two years I got to see the Pirates, my favorite NL team (and my Dad's), in spring training. (My favorite AL team was the Tigers where I saw my first professional baseball games a few years earlier.). In any case, that was the beginning of my dislike of the Mets.
A couple of years later we moved again, this time to Texas. My Dad's Pittsburgh roots ran deep, but as a brand new Texan I continued to follow the Astros neé Colts. Probably due to the successes in 1968, 1979 & 1984 of my "favorite teams", the Astros were always a bit in the background, but seeing several games in the Dome helped. And I was definitely crushed by the collapse of JR in 1980. But I think 1986 is where my "bandwagon" turned the corner and I became an Astros fan first, with only a nostalgic attraction remaining for the Pirates and Tigers. And when the Express showed up in RR, I was among the first to buy season tickets. I had always followed the minor leagues, but that was when I graduated from occasional bus rider to aspiring bus driver. Since 2000 I have spent 100% of my baseball time on the Astros and their minor league system. I barely even recognize the names on opposing teams unless they happen to be refugees from Lexington or Salem or Lancaster or somewhere like that. I am probably missing a lot of good baseball, bit I am enjoying the heck out of this "Golden Age" ... and all the more for my time on the bus.