The widespread and common narrative that Khadijah (ra) was 40 years old when she married the Prophet (saw) is most likely weak/fabricated, due to most of these narrations being reported by Muhammad ibn Umar Al-Waqidi

Al Waqidi was known to be a fabricator of hadith and a storyteller. Imam Al Shafii said, "All the books of al-Waqidi are lies. In Medina there were seven men who used to fabricate authorities, one of which was al-Waqidi." Also Al Dhahabi in his book Mizan al Itidal...

has compiled statements of those of the muhadditheen such as Al Bukhari who says, "al-Waqidi has been abandoned in hadith. He fabricates hadith". Additionally, Ibn Hajar al Asqalani in his Tahdhib al Tahdhib said, "He has been abandoned in spite of vastness of his knowledge".

These are some of the many statements from classical scholars, which especially came from those of the muhadditheen, who have called Al Waqidi a known major liar and fabricator of hadith.

A thing to note is that one reason of why Khadijah (ra) being 40 is widespread is possibly due to past scholars simply narrating it in their books, either in neutrality, in doubt or sometimes even being in favour of it i.e. Ibn Al Jawzi, Ibn Kathir, Ibn Abd al-Barr & others

Additionally there is one narration which suggests that Khadijah (ra) was 28, however this narration is much weaker due to the unreliability of two narrators in the chain, Hisham bin al Kalbi and his father Muhammad bin al-Kalbi, in addition to far less scholars reporting it

Therefore, it can be concluded that there are no Sahih narrations mentioning the actual age of when Khadijah (ra) married the Prophet (saw) & her age is unknown

(Sidenote, the narrative of Khadijah RA being 40 has also been used to push certain agendas related to the marriage)