You could achieve this in two ways..
1. Using 'Required' Field User Property
Setting this property to TRUE, made the field mandatory while saving the record by evaluating the expression..
Create a calculated field with following value
Calculate Field Name : Calc
Calculated field value: IIf((Today() - [Created]) >= 5, 'Y', 'N') and see..
if (this.GetFieldValue("Calc") == "Y")
TheApplication().RaiseErrorText("Field is a mandatory field");