I had Sprite done as early as they would allow - think it was about a year or 18 months. I think they grow too much before then, so doesn't give a clear mark if done too early.
They can be done from 1 year old, I had Herbie done pretty much on his birthday, I called it his B'day present, he was also micro-chipped that day and I don't think was impressed by his present at all!!!