This wiki answer mentions, that Khadija (RA) the business women and first wife of the prophet could be 28 years old, not 40 at the time of her marriage with the prophet
if you google for "The Sons of Khadija" by M. J. Kister (1993), you will find interesting evidence that she was actually only about 28.That makes sense when you consider that she bore Mohammed six children.
I am asking this because even the age of Ayesh (RA) is disputed, but we never dispute Khadija (RA) age. How can we be sure that she was 40 years old at the time of marriage? Is there any reference to this fact?