Keep in mind what I'll write in just my experience not a you should...
I think by 18 months we knew. I bought my first gt book by 2. But I'm thankful we didn't test earlier than 4. Whatever we were doing must have been fine. They were happy and so was I. We did lots of crafts and adventures and books. I didn't know anything about kids so I never took the age limits on products seriously (in terms of puzzles etc.). For the first 6 months after being tagged hg+, I stressed out about "enrichment" and "education". Keep in mind, I don't work so a little pre-schooling for social benefit was fine (if it was to be 5 days a week... we would have needed more).
But I stick with I don't want any more tags on the kids until I need the proof for school. There are many times I wish we didn't find out at all ;-) but formal schooling would have been a shock if we didn't learn exactly how different the kids are. but for me, i'm glad i just wondered without stress.
