From al-Malfudh Sharif, pg 350 – 351:

Question
If there are women performing salāh and behind theme there is a wall. What is the condition of the salāh of the men standing behind this wall?

Answer
If the wall is so low that the men are able to see the heads or chest of the women in front, then salāh [performed by the men] is not accepted.

Question
What if the women are elderly?

Answer
Old or young, it is forbidden for women to go to the Masjid. It is stated in a blessed Hadith, “For women, salāh performed in a [secluded] cellar is better than that performed in their [private] room. Salāh performed in her room is better than that performed by her in the veranda [of her home]. Salāh in her veranda is better than the salāh performed in the courtyard [of her home]. And the salāh performed in the courtyard is better than the salāh performed in my Masjid [Masjid al-Nabawi Sharif]." [1]

Thus, women are exempted from performing salāh with the congregation in a Masjid. In fact, it is forbidden by the Sharī’āh for women to perform salāh in a Masjid. Their association and intermingling with men is never empty of Fitnah. Look around you today! What is the moral situation of the Muslim society? Does this not open the eyes of the Muslim?

[1] In this hadîth, Sayyiduna RasulAllah şallAllāhu álayhi wa sallam emphasises that the more a woman hides herself in performing salāh, the better and more excellent it is for her