It's never too late. Some people only tag them at sale time, if needed. Others like us do theirs shortly after birth or within a few days, when they're easier to catch and handle. Older sheep that have lost their tags can be redone also, using the same hole if it's not torn, or a new one if it is.
Trust me, the younger the better.
Peg Haese, PNP Katahdins, far SW Wisconsin USA