I wouldnt write any child off before a certain time, they may start to fly in year before exam.
however I am sure you can tell the dc whose brains are working quickly and easily on subjects...from year 1 and 2.
My dd started to fly in year one, and before that her progress compared to her peers was slower.
But whilst I always read to her, exposed to book and so on, I didn't school her from early years like some peers. I heard them saying at 3
" at least he holds the pen properly now" and I remembered going home and once again trying to train her to hold the pen but she didn't get it.
Some peers parents had them knowing numbers, alphabet, writing name before nursery.
So you would have said comparing her with them, she wasnt 11+ material, but now, she is storming ahead.
Her writing is beautiful, quick at Maths, amazing reader, vocab brilliant, natural aptitude for spelling, we don't even do her spellings properly at HW because once she sees a word she seems to remember how to spell it.
So NOW I would say, so far ( as long as her enthusiasm keeps up) she is 11+ material.
So those that are showing no signs before exam wouldn't write off, but certainly ones like DD who have clicked now, I would say have a good chance at it.