Hi Dan...Is your FD getting enough light? (direct sun)Also, do you fertilize, and if so what type of food? Toni

I don't believe anyone has yet found a way to make juvenile citrus (or Poncirus) seedlings flower and fruit early.
In the case of Poncirus, including F.D., here in the UK they usually start flowering around 12 years old. Probably varies a bit in different climates, and the occassional plant does flower much earlier for no apparent reason.
Yes, your plant will flower. Just be prepared to talk to it for another 10 years!
Has anyone ever seen a variegated Trifoliate orange? They do exist, but I've never managed to find one yet.

It never occured to me to do this, because I can't immagine wanting the FD fruit, but if you really want fruit, you could graft mature FD bud wood onto the immature FD root stock.

They average 5 years to fruit in SE Texas.

How do you know it's a Poncirus "flying Dragon"? How does it look like? Citrus trees grew wild in my garden. They often bloomed at 12 year old or older. Their fruits were so dry that I had to cut the trees off. Now I think that I was so stupid, I could have used them as root stocks.

Flying Dragon is easily distinguished from all other Poncirus varieties by
1. stems that are quite zig-zag in their growth; they change directions at every leaf
2. thorns are strongly curved, never straight
3. trees grow at perhaps half the rate of other P.t.

They do produce nucellar (true-to-type) seedlings, but also produce a high percentage of zygotic (not true-to-type) seedlings, so if you're growing them for rootstock use, it's important to carefully rogue the seed bed for off-types.

A 4-foot plant may be almost big enough to flower. I'm thinking we've flowered them at about 5-6 feet, from seed.