A women may not lead a man (whether they are old or young) in salat at all. Though it is fine for a women to lead women in salat, and there is some differences among scholars in this. There is a Hadith:
لاَ تَؤُمَّنَّ امْرَأَةٌ رَجُلاً
No woman should be appointed as Imam over a man
But this hadith is not authentic and is weak, but there are authentic Ahadeeth which support the meaning, the Prophet (Peace and Blessings be upon him) said:
لَنْ يُفْلِحَ قَوْمٌ وَلَّوْا أَمْرَهُمُ امْرَأَةً
Never will succeed such a nation as makes a woman their ruler
صحيح بخاري (Bukari)
Another is, The Prophet (peace and blessings be upon him) said:
خير صفوف النساء آخرها
the best of the woman's row is the last
صحيح مسلم (Muslim)
Ibn Uthaymeen said:
And this is proof that women have no place in the front, and the Imam
is in the front, so if the Imamat of Women is correct then the
position of the women would still not be flipped, so that would mean
that a women would have been put in front of the men and that is not
supported by the Shariah.
الشرح الممتع صـ312و313