mental age

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Wiktionary

1. n. How old one appears to be based solely on mental ability and function.

WordNet 3.0

1. n. the level of intellectual development as measured by an intelligence test