First time I broke 90 was summer 1994, my last day of 8th grade. We had a half day of school and I went to the course that afternoon with friends. I'd been playing pretty well that spring, but that day things really clicked and I eliminated a lot of the big numbers and shot 82, which is a hell of a first sub-90 score. A week later I had surgery and didn't play most of the summer, but it didn't take long to get back.
First time I broke 80 was a year and change later. I had been edging close but never quite able to do it, especially because I had a weird habit at that time of starting terribly and then playing well on the back nine--but I generally had dug myself too big a hole to be able to make it work. Shortly before golf team tryouts that year (entering my sophomore year - in CO, fall is boys golf season) I played a course in Loveland called Mariana Butte and, in typical fashion, shot a brutal 43 (+7) going out, which would mean I would have to play the back even to break 80. And in true fashion, I did get my game together on the back and made pars from 10-16 ... and then doubled the 17th. 81. Oh well.
The next day we played a course called Legacy Ridge. I was even worse on the front side that day (+8), including a double on the ninth that had me steaming. But again, I got it together on the back and played even through 14. Then I made a sloppy bogey at 15 and figured that was it. I have never made a lot of birdies and certainly didn't back then--plus the closing holes at Legacy Ridge were marketed as long and tough (really 18 isn't so long, but 16 and 17 are). Maybe it was that the pressure was suddenly off and I wasn't thinking about trying to shoot 79 anymore, but I hit a really nice long iron on 16 and drained the birdie putt. Then on 17 I hit a great drive and another good long iron and made the putt again (this was also my first time making consecutive birdies). Suddenly I needed a par at the 18th (a par 5) for 79. I hit two good shots and then hit a horrible wedge into the left bunker. But I got it up and down for my first sub-80 round.
And, as several others have mentioned, I then rolled into tryouts later that week talking up how I'd shot 79 ... and suddenly looked like I couldn't break 90 to save my life. Golf, man.
Finally, I've broken 70 a few times. The first was summer 2002. I was playing a Tucson muni called El Rio, which is a par 70. I cam to 18 needing a birdie for 69. 18 is a short par 5, but a severe dogleg right around the driving range and a huge net prevents you from trying to cut the corner, so it doesn't play as short. I hit a good drive that worked around the dogleg a bit so I actually was able to get my second shot right up in front of the green and left a pretty simple little chip. Needing to get it down in two, I holed the chip for eagle and 68. Pretty great feeling.
The goal I really have now (because breaking 60 is not happening) is a bogey-free round. I've had a couple rounds with just one--including one round where I didn't make a bogey until 18, which would have been worse except I shot 68 (4 under) and that cures a lot of bad feelings.