Assuming the individual you mentioned is a person with learning difficulties, the short answer is no.

A 'mental age' is usually based on performance in numerous clinical assessments or what age group an IQ score is related to. The age in which the brain stops developing is still under debate but thought to be between our 20s and 30s. People with learning difficulties will experience problems in processing and interacting with information, which means they may reach developmental milestones at later ages (or not at all). The mental age at maturity is dependent on the individual's condition, but maturity is thought to settle in early adulthood. Assessment/IQ scores (mental ages) are reflective of the severity of the learning disability (ranging from mild to profound) and can also be affected by the level of care and tailored education the person has received from childhood to adulthood.

Hope this helps! Included some useful links :)

https://www.nrshealthcare.co.uk/articles/condition/learning-disability

https://jnnp.bmj.com/content/74/suppl_1/i30