It's easy to tell how old you are chronologically...if you are 10 years old, then your chronological age is 10. But determing a person's mental age, or the age-level at which they function mentally or intellectually, is much harder. Binet actually developed tests to measure mental age and it measures a person's level of performance as it corresponds to their chronological age. For example, one child age 10 may be able do sophisticated math, read more effectively, and comprehend more than another child the same age. In this case we might say that one child has a mental age greater than the other and is more in line cognitively with people several years older.