I tattoo when I wean at about 10-12 weeks, I like to do them at 8-12 weeks, after that they grow considerably thicker, I find the younger I do it the better the tattoo stays in the ear,
WARNING GRAFFIC:
this sounds rather harsh but if using a clamp puncture hard enough so you have to peel the ear from the tattoo digits, At a younger age the ear cartilage is softer so its not as painful, its like getting your ear pieced on the lobe, its soft tissue. Now if you wait until the rabbit is older and the cartilage is tough it will be more painful and heal slower. And if you don't puncture hard enough so that the tattoo stays you will have to tattoo again, so its a double ouch for the rabbit. Also when you do it at a younger age you can restrain your giants better.