At what age can you diagnose a child with a learning disability?

Depends on issue. A learning disability implies a child with at least average intelligence shows a lack of ability in a specific area compaired to other kids their age. A reading disability will become evident when other kids can & they struggle. The same with math, remembering sequences of items, etc. Kids brains mature over time & new skills come in at variable rates, some have simple delays, others real problems.
Most 5-year-olds can. participate in standardized testing of IQ, processing & achievement. Persistent Global Developmental Delay is then diagnosed as Intellectual Disability if IQ is 70-75 or .
Poor performance. a leaning disability implies that a child is not meeting expected performance level in school based on intellectual expectations (IQ). It excludes mental retardation which is another topic altogether. You cannot make a diagnosis of LD until a child has reached the age and academic point where they are exposed to the material.
LD. Generally, diagnosis of learning disabilities is best in the 8 to 10 year age range, since more independence of work is required, and you are likely to see difficulties at that time.