Given that you cannot modify the data, I assume you are looking for data analysis techniques that can highlight a systematic pattern? If so we need more info.
Make use of context of the problem. Is the product young children or grown adults could buy? How was the data collected? Could some entries be from scammers or legacy software bugs? What fraction of inconsistent records do these represent? What other columns do you have about these customers? It really depends..
For example: If the data was manually entered or digitally scanned from physical documents it’s possible either year entry had “butter finger” or OCR translation errors. Perhaps you can semi infer other customers as family members based on their last name & other demographics. Perhaps clustering all customers reveals well defined sub groups between consistent & inconsistent records.
Be creative, formulate hypothesis & attempt to prove them with data to tell a plausible story. Make sure to analyze the consistent customers too in order to reject some hypothesis.