Hi,
The 15mm is the standard size, this can be used anytime up to slaughter, but I would wait until the pigs are slightly older before using 15mm - I can't be precise with the age as I have no idea how big your pigs are - but I would suggest that it would be OK from around 12 weeks onwards.
As a matter of interest, the reason you can't slapmark before 10 weeks is that the piglets body is very efficient when it is very young, and as such will "Cleanse" even permanent Ink from the skin / fat (But not from cartilage)
Thanks