40 years of age is a very important milestone in the life of a man (woman). Keeping in mind the average lifespan of sixty-seventy years, one who reaches 40 years of age completes about two-thirds of his (her) average lifespan; thereby meaning that he (she) enters into the last third of his (her) life.
Allah has emphasized this stage of human life very clearly in Qur’an. Please go through Aayah No. 15, Surah Al-Ahqaf, Chapter No. 46, Holy Qur’an.
We have enjoined man to treat his parents with kindness. With much trouble his mother bore him, and much pain did she give him birth. His bearing and his weaning took thirty months. When he reaches the age of full strength and becomes forty years old, he says: “My Rabb! Grant me the grace that I may thank you for the favors which You have bestowed on me and on my parents, and that I may do good deeds that will please You, and grant me good children. Surely I turn to You in repentance and surely I am of those who are Muslims.”