Here are two fruit on a seedling flying dragon in a 5 gallon pot a few years old:

how old was it when it fruited does fd fall in the 3 to 5 year time frame ?

Cool, how tall is it? I have one or two I will probably let grow for a seed tree.

I really don't like how slow growing FD is though. You pretty much have to have it in it's second year to be ready for rootstock.

Satsuma seedlings are really slow growing. I have one that is 18 months old and it is only about 12 inches tall! Maybe next year it will grow faster.

I have a couple of citrus aurantium seedlings. At what age can I expect the first flowers on them?

Maybe 5 years in sunny Arizona.
More like 15 years in the UK (and Belgium)!

Regarding PT FD, I have several seedlings about 12 years old. Only one has produced a few flowers, and no fruit yet.

I have a standard Trifoliata that fruits the first year from seed.

So you have a mature Poncirus plant that produces fruits with seeds that themselves produce seedlings which generally flower and fruit in their first year?
How tall are the seedlings when they flower?
Are the flowers from a single terminal bud?
Are the seedling fruits normal sized and contain seeds?
Do you know if this is maintained through further generations?

Citrange, you make many good points.
Also consider an early fruiting seedling, say at 2 years, that consistently fruits every year from then on. This could be distributed or maintained as cuttings or budwood etc, as is usual with all other Citrus cultivars. I agree that transmitting the early fruiting to its seedlings would be an ultimate test.
However a variable seedling may also produce further variable seedlings, as is sometimes seen in monoembryonic Citrus seedlings.

For those in Arizona, is the tap water very salty? Do you amend the soil before planting? Southern Arizona is very very dry. Do you irrigate with tap water?