So what is the earliest concert you can remember going to

John Denver in 1976, but I don't claim that.

My uncle and I had a tradition of going to Carl Casper's custom auto show that started in the early/mid 70's.
In '79 he thought I was old enough to stay for the band which was Brownsville Station that year. I was 11.
I thought it was too loud and we went on about the show, but he convinced me to go back in time to hear "Smokin In The Boys Room".
I do claim that.

The first arena rock show was Judas Priest/Iron Maiden 1982 (Screaming For Vengence/Number Of The Beast).
That is the high standard by which I judge all other concerts.