Renewing your repentance after one reaches 40 years

وَوَصَّيْنَا الإِنسَـنَ بِوَلِدَيْهِ إِحْسَـناً حَمَلَتْهُ أُمُّهُ كُرْهاً وَوَضَعَتْهُ كُرْهاً وَحَمْلُهُ وَفِصَـلُهُ ثَلاَثُونَ شَهْراً حَتَّى إِذَا بَلَغَ أَشُدَّهُ وَبَلَغَ أَرْبَعِينَ سَنَةً قَالَ رَبِّ أَوْزِعْنِى أَنْ أَشكُرَ نِعْمَتَكَ الَّتِى أَنْعَمْتَ عَلَىَّ وَعَلَى وَلِدَىَّ وَأَنْ أَعْمَلَ صَـلِحاً تَرْضَـهُ وَأَصْلِحْ لِى فِى ذُرِّيَّتِى إِنَّى تُبْتُ إِلَيْكَ وَإِنِّى مِنَ الْمُسْلِمِينَ - أُوْلَـئِكَ الَّذِينَ نَتَقَبَّلُ عَنْهُمْ أَحْسَنَ مَا عَمِلُواْ وَنَتَجَاوَزُ عَن سَيْئَـتِهِمْ فِى أَصْحَـبِ الْجَنَّةِ وَعْدَ الصِّدْقِ الَّذِى كَانُواْ يُوعَدُونَ

(15. And We have enjoined on man to be dutiful and kind to his parents. His mother bears him with hardship. And she delivers him with hardship. And (the period of) his gestation and the weaning of him is thirty months, till when he attains full strength and reaches forty years, he says: "My Lord! Grant me the power and ability that I may be grateful for Your favor which You have bestowed upon me and upon my parents, and that I may do righteous good deeds, such as please You, and make my offspring good. Truly, I have turned to You in repentance, and truly, I am one of the Muslims.'') (16. They are those from whom We shall accept the best of their deeds and overlook their evil deeds. (They shall be) among the dwellers of Paradise. That is a promise of truth that they had been promised.)
Surah Al Ahqaf - (15-16)

Tafseeer Ibn Katheer. (Relevant lines pertaining to repentance as one reaches the age of forty)

"and his gestation and weaning is thirty months, till when he attains full strength";
meaning, he becomes strong, youthful, and attains full ability. (and reaches forty years,) meaning, his complete intellect, understanding, and patience reach the level of maturity. It has also been said that usually one will not change his ways once he reaches the age of forty.

"he says: "My Lord! Grant me the power and ability ;
meaning, `inspire me.'

(that I may be grateful for Your favor which You have bestowed upon me and upon my parents, and that I may do righteous good deeds, such as please You,)
meaning, in the future.

(and make my offspring good.)
meaning, my offspring and descendants.

(Truly, I have turned to You in repentance, and truly, I am one of the Muslims.)
This contains an instruction for anyone who reaches forty years of age to renew his repentance and turn to Allah with strong resolution.

(Those are the ones from whom We shall accept the best of what they did and overlook their evil deeds. (They shall be) among the dwellers of Paradise.)
meaning, those to whom applies the above description -- those who repent to Allah, turn back to Him, and rectify their shortcomings through repentance and seeking forgiveness - those are the ones from whom We will accept the best of what they did and overlook their evil deeds. Many of their errors will be forgiven and Allah will accept from them a modest amount of deeds.