I have read graft 12 hour old larvae. How does one make that determination?
Well explained by Brother Adam in his book Beekeeping at Buckfast Abbey. Place a warmed frame into the breeder colony. Twice a day you lift the frame and check for eggs. The first time you see eggs, now you can start the clock on when it's time to graft.
I use a slightly modified version of that. I graft on Wednesday, so Sunday I go thru the colony I intend to graft from, find a frame with fresh eggs that are still standing, more often than not I'll see the queen on that frame still layng. Write down which frame it is, then when I go to graft on Wednesday, pick out that frame and brush off the bees. It will have plenty of correct age larvae, and likely some still eggs.
I used to struggle to see the tiny correct age ones, but wife gave me a set of jewelers loupes for christmas a couple years back. Plenty of magnification with an led light made all the difference in the world. Not only can I see the larvae before I pick it up, I can see it in the jelly after placing in the cup, and those that dont work out I just do another one into that cup. My take rate went from under 50% to over 80% when I started using those.
The other thing that made a big difference for me, not all grafting tools are created equal. The chinese tool is inexpensive, so buy a handful and try a bunch of them. All will have slightly different flexibility in the tip. I have one that works far better than the other 6 I've had for a few years, and I keep going back to it, it's the only one I can get those really young larvae with correctly. When we were at Apimondia, on the last day some of the booths were selling off inventory at a very good price so they didn't have to pack it up and take home. I bought a bag of 50 grafting tools at one of the vendors from China, dickered it down to 10 dollars at 3:30pm on the last day.