mental age

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A measure of mental development as determined by intelligence tests, generally restricted to children and persons with intellectual impairment and expressed as the age at which that level of development is typically attained.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun How old one appears to be based solely on mental ability and function.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the level of intellectual development as measured by an intelligence test