Fun going through the list. I remember one time when I was at ORD and still in my UA days, I looked up the tail number of the 757 I was about to board. Turned out it was the oldest one in the UA fleet at the time and I can't remember if it was also the first one ever delivered to UA.
Speaking of older planes, does AA still have anything flying with the old-style lavatories with swirling blue water versus the vaccum/pressure flush? UA still had quite a few old 757s that had them but it's actually been a really long time since I've been on an AA 757 and/or 767.