Taken from:
Gateway to Heaven (A Concise Manual for Muslim Sisters)
by Sayyid Shah Aal-e-Rasool Nazmi al-Husaini al-Qadiri Barakati Marehrawi
Sayyiduna Rasoolullah's (SallAllaho Alaihi wa Sallam) marriage with Sayyidah Khadija Radi ALLAHu Ta'ala Anha took place about fifteen years before the beginning of the Revelation, he being 25, and she was 40 years old.