The reason the Sul'dam are not collared as Damane and are able to pass that portion of the test is because although they have the potential to learn, they haven't begun learning yet. Aes Sedai cannot sense a woman's ability to learn to channel, only her inborn ability if she has not manifested yet, and even then she must be close. A woman who has to be taught to channel would have no sign, like the gray novice from the rebel camp (I'm drawing a blank on her name). She only had the ability to learn and until she started being taught she never channeled once by instinct, and she never Slowed the way channelers do. This leads me to believe that unless taught, they are indistinguishable from other women.

On that line of reasoning, women could be selected as future Sul'dam between the ages of 18-23 (25 being outside the norm, but still possible). The average novice training takes somewhere between 3 years (the record) and easily upwards of six. And this is for women actively recieving lessons and practicing the one power. The Sul'dam say that it takes a very long time for one of them to begin to see what a damane is channeling, so it would be safe to say that they don't learn enough fast enough to be detected before age 25. Also, none of the Sul'dam Slow the way channelers do. Every Sul'dam thus far in the series to be collared has been an older, more experienced Sul'dam, not a new/young one who hasn't had time to develop the sense.