Hadhrat Kahmash Bin  Hasan (Rahmatullah alayh) said: “When sin becomes old (that is, many years have passed since the sin was committed), most of you gain the idea that your sin has been forgiven. This is mere deception.”

While Allah Ta’ala forgives all sins, the incumbent requisite is to regret and sincerely repent. Most people who may have reformed themselves during later life, forget of the reckless life of flagrant sin they had led during their youth.
Their current ‘piety’ soothes their conscience and lulls them into the perception that their relationship with Allah Ta’ala has been rectified while this may not be so.
Remember that Allah Ta’ala does not forget.

It is incumbent to repent for all sins committed even decades ago. Mere reformation and practice of good deeds are not adequate for achieving forgiveness. It is incumbent to regret, confess one’s sins to Allah Ta’ala and to seek forgiveness from Him. Then, undoubtedly, He will forgive.
Allah Ta’ala revealed to Nabi Daawood (Alayhis salaam): “O Daawood! Say to Bani Israaeel: ‘You have abandoned regret and remorse. How do you know that I have forgiven you?

I take an oath by My Glory and Might! I shall reveal to you every one of your sins.’ ”