It seems a very simple discussion, but once you look both the attributes closely and study these, there’s a thin line leading to confusion.
Simple attributes are those that can no further be divided into further meaningful attributes. In other words, these are indecomposable. For instance; the First name is a simple attribute, where Name itself is a composite attribute. Where Name can be decomposed further into First Name, Middle Name, and Last Name.
Next is the single value attribute. A lot of websites quote examples like age, gender etc. A single value attribute also known as an atomic attribute has a value that would not change in a given period of time. For instance a serial number of a part. Age might fall into this category but preferably and easily it is a derived attribute depending on the computation of the date of birth and the current date.
Hope that clarifies!