Interesting but I got a completely different take on this. In the Post Crisis timeline Jay could not have originated on Earth Prime. Henry Allen existed there. In the new timeline parts of Earth 2 and 3 combined. Same as Earths 1 and 38, etc combined to create Earth Prime. As much as possible Jay's history on Earth 3 now takes place on new Earth 2. He was only on Prime after Crisis as a flaw in putting the new universe together. As we saw in characters on Batwoman.
Remember what Jay told Impulse - I am directly quoting Arrowverse wikia "When Jay first started to
time travel, he ended up fighting
Nazis in
World War II before living on and off in the past for 16 years.
"
This is a significant part of Jay's early life in the Post Crisis timeline. If we are using the important rule that Jay is a doppelgänger of Henry Allen and Barry Allen of Earth 90. He was born at same time. Most likely got his powers in 80s or 90s. Time traveled to the 40s, etc. a hold over from EArth 3 is that Jay had little life outside being the Flash. He did not meet Joan Williams until the 2010s. Which makes sense considering he spent a large part of his superhero career in the past with JSA.
On questions of age - clearly the speed force and other supernatural events slowed or stopped it. Similar to other members of the JSA. When we see Jay and Joan in Impulse and XS's future time they look the same age still.
This is mostly from memory! My favorite thing the Arrowverse ever did was casting John Wesley Shipp in all those roles. Particularly Stargirl which was subtlety supported on The Flash with his story of fighting Nazis.
