This study was in kids, but it found that while verbal IQs stabilized around 4th grade, non-verbal remained highly variable from year to year. I couldn't find a similar study for adults, but lack of evidence is not proof, and it's still very unclear whether IQ remains stable over a lifetime. Also, keep in mind that even if IQ scores remain constant, the raw scores consistently go up with age (and are subsequently adjusted to a standardized scale); so that it is very likely that a middle aged person of low-medium IQ can be more intelligent than a young person with high IQ. IQ also doesn't account for experience or knowledge (I would trust a low-IQ economist more than a high-IQ layman, for instance, in economic policy [assuming the education system is unbiased which is false in reality but the point stands]).
Also, .65 correlation with genetics means that slightly over 36% of the variation in IQ between people is due to genetics, I wouldn't say that's an amazing correlation.