What's the difference between developmental delay and mental impairment?

Specific testing: All children develop at different rates. Some learn to talk early (especially girls), some late, and some regress in language. Some develop rapid motor skills while others are much slower. Special testing can be done at school age to measure learning ability and to establish a scale of aptitude. This can then be used to quantify mental impairment.

Domain-specific : delay or Global Developmental Delay, significant delays in 2 or more domains, are diagnosed by standardized testing in kids birth-4 who have not attained expected milestones by the age at which 90% of typically-developing infants or toddlers have done so. Intellectual Disability is diagnosed in kids 5 & older by formal testing of Verbal & Non-Verbal IQ, whose Full Scale IQ is less than 70.