Twin studies of adult individuals have found a heritability of IQ between 57% and 73% with the most recent studies showing heritability for IQ as high as 80%. IQ goes from being weakly correlated with genetics for children, to being strongly correlated with genetics for late teens and adults. The heritability of IQ increases with age and reaches an asymptote at 18–20 years of age and continues at that level well into adulthood.
* https://en.wikipedia.org/wiki/Heritability_of_IQ
* https://onlinelibrary.wiley.com/doi/abs/10.1002/neu.10160
* https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4270739/
* https://www.cambridge.org/core/product/identifier/S183242741...
From that wiki article:
Twin studies of adult individuals have found a heritability of IQ between 57% and 73% with the most recent studies showing heritability for IQ as high as 80%. IQ goes from being weakly correlated with genetics for children, to being strongly correlated with genetics for late teens and adults. The heritability of IQ increases with age and reaches an asymptote at 18–20 years of age and continues at that level well into adulthood.