Do you want to make sure that the recipent of your product feed knows that your products are or maybe not are for adults?

This you’re able to tell by using the {adult} field.

Apply a simple search & replace rule like:

IF {adult} {is} [] THAN {replace value} OF {adult} WITH [no]

On the other hand – if your products are most for adults:

IF {adult} {is} [] THAN {replace value} OF {adult} WITH [yes]

You may be creative and use other fields to predict wether a product is ment for adults or not:

IF {description} {contains} [sex] THAN {replace value} OF {adult} WITH [yes]
IF {brand} {is} [Lego] THAN {replace value} OF {adult} WITH [no]