We have many different history regarding the marriage of Bibi Khadija (s.a). Many Shia historians do not accept that she was married before. They say she wasn’t 40 years old at her marriage time with Rasool Allah (saww). Some said she was just 2-3 years older that Rasool Allah. Even many Shia historians do not accept that Aisha was just 9 at her marriage time and they say she was at least 19-20 years old.
Even in Holy Prophet’s birth, mostly Sunni Historians believe 12 Rabi Ul awwal and Shia says 17 so when the personality Like Holy Prophet ( saww) around whom whole Islam is rostering, his both date is not one then don’t be amazed. It’s just Historical issue and doesn’t play any role in our faith. Our faith is that Lady Khadija ( a.s) was a pious Lady and was on Monotheistic religion. She never worshiped idols. A woman can’t be the mother of a Woman who is the leader of the women of the Paradise. Our Aimma’s mothers were all pure woman and selected from Allah (swt), they were not an ordinary woman.

Yes, there are many books ans many Shia scholars have reported and mentioned in their books that she was married and she was older than Prophet (saww).

But there is point that Muslims after passing 1400 years still we have not understood that In Islam or even in any Religion these factors have no role and in our character and personality. Our age, martial status. our wealth etc… has no role
Lady Mariyam was virgin and unmarried but she is one of the 4 chosen woman by Allah amongst all woman of the universe.
Prophet Isa and Prophet Yahiya Allah appointed them on Prophecy in their infancy.
Abuzar Ghaffari (r.a) amongst the all companions of Holy Prophet was the poorest but what status he has.

We should look the Sacrifices of this great Lady. We should look her devotion towards Allah while she was princess of Arabia but since she became the wife of Holy Prophet she considered herself as a slave woman of Rasool Allah. She use to cook bread and go to Cave of Hira in “Mount Noor” She left her palace and lived in “ cave of Abu Talib” we say Shabe Abi Talib for three years and after the social boycott of Kuffar of Mecca and because of that she passed away.
So even if other wives of Rasool Allah were virgin or young and not married but none can reach to the greatness and character of Lady Khadija (s.a) up to an atom weight.
Allah has chosen her amongst 4 woman who are the leader of all woman in this universe and greater than all the woman.