Have some data on that? This is what I've been able to find, and it suggests 'fluid intelligence' only drops down 10% from your peak in your 20s by age 54, and down 14% by age 64, which is nowhere near what you're suggesting (in fact it never gets that low according to this). And crystallized intelligence much slower than that.

"Fluid intelligence or fluid reasoning reflects the ability to solve novel problems, the kind that aren't taught in school," he explains, "whereas crystallized intelligence or crystallized knowledge measures learning and problem solving that are related to schooling and acculturation."

These different types of intelligence show different patterns as you get older.

Crystallized intelligence "averages 98 at ages 20–24, rises to 101 by ages 35–44, before declining to 100 (ages 45–54), then 98 (55–64), then 96 (65–69), then 93 (70–74), and 88 (75+)," says Kaufman.

Fluid intelligence drops much more quickly. Kaufman reveals that it "peaks at ages 20–24 (100), drops gradually to 99 (25–34) and 96 (35–44) before starting a rollercoaster plunge to 91 (45–54), 86 (55–64), 83 (65–69), 79 (70–74), and 72 (75+)."

[1]: https://www.sciencealert.com/does-iq-decline-as-we-age-one-t...