To my knowledge, there is no reason why a woman, who has been a mother, cannot become a religious (nun or sister) as long as her children are grown and able to live on their own and provide for themselves. It is by no means common, but there doesn’t seem to be any reason why not. Obviously, if she had been married, she would have to be widowed (or, hypothetically, granted a decree of nullity).
However, to do as the OP suggests, and adopt children, presumably see them through to adulthood, and then seek to enter a convent, that would both be highly out of the ordinary, and would raise a question that begs to be answered, “why are you adopting now, with the eye towards going into the convent in twenty-odd years?”.
If you feel as though you are truly called to motherhood, yet do not wish to marry, my advice would be to focus on that vocation, and what might happen twenty years from now, well, that’s a long way off. I would add that deliberately becoming a single parent has a huge set of challenges of its own. Ideally, children need both parents, husband and wife, in the same home.
the monastery I want to join would allow me to join until 45 (it’s my dream monastery 
). I emailed them my question and will wait for a response. If they say yes or no, I will take it as the will of God. I feel Mary has been leading me to go there through many prayers.
