When I spoke to Freeemark about my foal (10 months at the time), they said they won't do them on anything under 12 months, and that somewhere between 12 and 18 months is their minimum. Don't know if Farmkey have a different policy though.
Farmkey say from a year - I had Stinky done at 16 months and had to have the last two numbers touched up as he grew a lot - they did this last Saturday and you could see how much he had grown.
I still would get it done as soon as I could given the choice of waiting until he was grown or having it retouched later.
I will have to wait a little while then as she is only 11 months old. I would rather not have to get it re-done either so will probably wait until she has grown some more!