I tattoo at 8 weeks old, depending on what form of tattooing I'm doing. Clamp I tend to wait at least 3 and half months for my keepers because I find it's easier. Since I have really shaky hands my husband has to do the pen tattoos for me and he generlly will do those at 8-9 weeks. I breed Mini rex, So I'm sure it really differs on breeds.
I did find latley that the older the rabbit, the more tolerable they were to being tattooed. the other day, I tattooed a 7 month old buck and he barley flinched, then I tattoed a four month old and she really had a fuss about it. I have seen that in my others too.
I think fading depends on how deep the tatoo went and the type of ink.