It's not that older engineers are worse at them (from an intelligence standpoint), it's that younger engineers have more time to study for them.
Take a standardized test like the LSAT. Young single people can spend 4-5 hours a day taking practice LSAT exams. Older married people with small children barely have 2 spare hours after work let alone 4 (without neglecting their spouse/kids).
Assuming both the older and younger devs in this scenario have identical IQs, who is likely to do better come LSAT test day? It's a war of time attrition.