You should read my post past the first paragraph. I explained my reasoning for why the Sul'dam would pass that initial test of being collared up to 25. She wouldn't have learned enough to be considered a "channeler" of sorts until many more years of being Sul'dam.

For the sake of putting numbers to it lets say this:

A woman who has the spark manifests around 18.

A woman who can be taught manifests sometime after she begins learning.

A woman learning the power gets proficient between 3 and six years, so now our woman is approx 21-24. This is for someone actively learning to channel.

It would be a reasonable assumption that a woman observing the one power in the belief she cannot use it would require MUCH longer to train, making her AT LEAST 30 before she can begin to see weaves, and when her ability would be able to be sensed or controlled by the a'dam.

Every woman tries on the collar first, then the bracelet.

The Great Hunt
Ch. 46 To Come Out Of The Shadow

Egwene speaking: "Not just any woman can become a-a Leash Holder. Renna thought she was being friendly telling me about it. It is apparently a feastday in Seanchan villages when the sul'dam come to test the girls. They want to find any like you and me, and leash them, but they let all the others put on a bracelet to see if they can feel what the poor woman in the collar feels. Those who can are taken away to be trained sul'dam. They are the women who could be taught."

Collars first, bracelets second, until 25. Sul'dam manifest much past 25.

I will concede that it may be possible once a woman has tested positive to become sul'dam that she is never tested again whether or not she is 25 yet, but I do not know. I think my theory is a good one either way.