This refers to different types of psychological assessment. The "verbal" refers to tests that use verbal materials, e.g. the verbal fluency tests on the Wechsler Intelligence Test for Children. These tests include finding commonalities between words like "How are red and blue similar?". In contrast, "non-verbal" tests require reasoning with visual materials. An example for this is the Raven Matrices test, where a visual pattern has to be completed by the participant. 

The other part of the question is about mental age. In developmental samples, the performance of a person is often expressed relative to the performance of a large sample of typically developing children of the same age. If the participant performs within the average range compared to the normative sample, then the mental age matches the chronological age. If she/he performs above or below, the performance can be compared to the average performance of sample older or younger than her/him. In other words, a younger mental age means that the participant performance at a level expected for children younger than him or her. 

This refers to different types of psychological assessment. The "verbal" refers to tests that use verbal materials, e.g. the verbal fluency tests on the Wechsler Intelligence Test for Children. These tests include finding commonalities between words like "How are red and blue similar?". In contrast, "non-verbal" tests require reasoning with visual materials. An example for this is the Raven Matrices test, where a visual pattern has to be completed by the participant. 

The other part of the question is about mental age. In developmental samples, the performance of a person is often expressed relative to the performance of a large sample of typically developing children of the same age. If the participant performs within the average range compared to the normative sample, then the mental age matches the chronological age. If she/he performs above or below, the performance can be compared to the average performance of sample older or younger than her/him. In other words, a younger mental age means that the participant performance at a level expected for children younger than him or her. 

thank you very much and just wondering whether to use non-verbal mental age matched to test vocabulary development of children with down syndrome  or to use verbal mental age matched td children

Both verbal and non-verbal tests are commonly used and there are plenty of well standardised assessments for both. The choice depends on the research or clinical question that you are trying to address. 

thanks joe

As I see it, non-verbal mental age and verbal mental age refer to two different types of one's intelligence. Whereas the latter has to do with one 's ability to master all that is related to language (e.g., spelling, grammar, semantics, and the like), the former has to do with one's ability to perform certain behaviors, such as to solve a mathematical or logical problem. Note that both WAIS and WISC allows us to determine an verbal IQ and a performance IQ.

Although distinct, these two abilities are intertwined. Suffice it to say that one's proficiency in an given language involves a set of overt behaviors, and that it is often the case that one's private speech lies at the heart of an overt behavior (e.g., to solve aloud a mathematical problem). Self-instructions are a typical procedure to solve everyday problems.  

I hope that I have provided you with some hints on the difference (and resemblances) between verbal-mental age and non-verbal mental age.

Best regards