Aadhaar is not mandatory for bank accounts. It never was. I have not attached aadhaar to any bank account except one and other than that Aadhaar linked only to PAN.
Now re-KYC is required by banks as per RBI norms. People don't generally have information on why they are asked for re-KYC. Like you said you have 15 years old account still active. Maybe you are using it for say 1-10 txns or within limits per year and also your txns are within the KYC data bank already has feeded incl. annual income etc. They will ask for re-KYC only when your txn patterns of say cash deposits, online/offline withdrawls, ecom/pos txn crosses the declared info. Only then they will bug you have re-KYC and update information like annual income, source of income.
Customer can't do much as it is RBI policy to have re-KYC every two years only when bank account txn pattern changes otherwise not required. ICICI never asked me for re-KYC but in 2019 my account was being used more and unusual cash deposits as normally it was in thousands per txn and then it was in lakhs. So they froxe my account. I went to branch and did re-KYC and it was reactivated instantly. They sent me SMS and Email regarding same as to why they need to do re-KYC and I already know why system got triggered. 🙂