In my Sociocultural Theory classes the focus recently has been on Vygotsky's Zone of Proximal Development (ZPD). The ZPD is the difference between what someone can learn by themselves, known as the Actual Development Level (ADL), and what they can learn with assistance.

Say for example you have two 10 year old students who when working by themselves show the mental age of an 8 year-old, meaning they can successfully work out problems that have been standardised to the 8 year-old level. It is important to note that the ADL has the prerequisites of no assistance (i.e through leading-questions, or showing the problem being solved); the students need to prove that they can do it by themselves. You don't stop there however. Following this, you give the students help in various ways to help answer a problem. It may then turn out that child A's mental age with assistance is 12, whereas child B's may be 9.  Does this mean that they are at the same mental level? The difference between 12 and 8, and 9 and 8 respectively is what is known as the Zone of Proximal Development. 

"The zone of proximal development defines those functions that have not yet matured but are in the process of maturation, functions that will mature tomorrow but are currently in an embryonic state" (Vygotsky, 1978, p. 86).

Therefore to truly know the mental age of a student we have to find out the ADL and ZPD. It is also important to add that what is in the ZPD today will be in the ADL tomorrow.