The last scientific results (fRMI) say that brain develops till 40 (December, 2010)

Quran says :

"15. And We have enjoined upon man, to his parents, good treatment. His mother carried him with hardship and gave birth to him with hardship, and his gestation and weaning [period] is thirty months. [He grows] until, when he reaches maturity and reaches [the age of] forty years, he says, "My Lord, enable me to be grateful for Your favor which You have bestowed upon me and upon my parents and to work righteousness of which You will approve and make righteous for me my offspring. Indeed, I have repented to You, and indeed, I am of the Muslims." 16. Those are the ones from whom We will accept the best of what they did and overlook their misdeeds, [their being] among the companions of Paradise. [That is] the promise of truth which they had been promised." (Quran, 46: 15-16)