Three engineers and one age field

A product manager comes up with an idea: add a new attribute to capture the user's age and display the number. At a high level, the requirement might be met by one of three kinds of engineers.

Engineer 1: “Sure, I'll get it done before lunch.”

Engineer 2: “What are we going to store in it? What validations do we need? Can this attribute be updated, retrieved or deleted, and if so, who can do that? Is this PII? What's the retention period? Does it need to be available in the data lake?”

Engineer 3: “Can we just derive it from the date of birth we're already storing?”

A thought exercise: which approach is most beneficial, and which one do you think the business will go with?