'Atha' indicates that before this something else was there, which obviously would be a treatise on life in general, as only when you have led life in general would you now want to go beyond it. So a lack of knowledge on life as it should be led, of which the four ashrams are an important part, would prevent one from entering the yog ashram. So now that you have led your life, so here I begin the discipline of yog, without which you are doomed to be stuck in the cycle of birth and death.

Discipline would mean a strict way of life, the sutras indicate the shastra to be a way of life. So a discipline seems important for achieving the final frontier....that is after the stages of life are crossed, which further could be in any life time...even this. Age of the physical body is not important here, it could be 8 or 80...it is just that are you ready?