Actually IQ has fairly high test-retest reliability. Obviously it's not perfect, nothing is, but it's still pretty good
have tests on that been done with intervals like 20 years ?
I tried to find an example, but could only find this one done on 344 students with a 2.84 year interval.
"Long-term stability of the Wechsler Intelligence Scale for Children--Fourth Edition"
2.84 year is long term ???
https://www.ncbi.nlm.nih.gov/pubmed/23397927