I have experienced the horrors of the question I believe it was the Ocala National show early 2020 I believe. Probably late February or March.
I carried at that time 3 spares in addition to the 4 on the ground. For that trip the four on the ground were at the beginning of their 5th year and I thought since it was "winter" heat wouldn't be a problem and could change them when back from that show, as I have had several trailer and tow truck tire issues before. The tires on the trailer were Marathon's and the spares were two Carlisle and one tow master I dont remember the date of the carlisles but the tow master was 7 years and was only used about 500 or so miles. I use a tire monitoring system but it wasn't any help on this particular show. I also carefully keep the tires to correct inflation. I'll admit this was prior to realizing there is a max speed on trailer tires. Since, I am a reformed tower.
I busted 3 tires that were on the trailer and had to stop and buy some tires in NC as was getting low on spares. anyway on the way back I had one original 4 year old marathon on the ground and the one 7 year old barely used Towmaster. well did ok until in NC on the way back on I-95 and blew out the last marathon and put my last spare on. That tire lasted probably 100 miles before it blew and this one exploded with such force it tore up 4 panels of my trailer, the fender and damaged the tire wheel well but didn't penetrate into the trailer.
So. I would never consider using your 7 year old tire.
My tire dealer told me that cracks on the exterior don't always appear on trailer tires but there is a way to determine if they might be ready to blow. The tread will slightly crown up in the middle of the tire or a "golf ball" will appear on the sidewall. And I am a reformed towing person now limiting towing speeds to 70 MPH. The new Endurance series of tires are the latest design and the max speed is about 80 I think. Never popped one of them though.
Robert