Intelligence as measured by IQ tests peaks around age 20 to 40 and then heads downward. But IQ is adjusted for age. By definition, the average IQ of 20 year olds would be 100. The average for 50 year olds is also 100, as is the average for 80 year olds. This is even though people get stupider and score lower on the tests as they get older.
Alan Kaufman, a researcher at Yale, calculates a "full scale" (global) IQ, where he removes the adjustment for age, and adds an adjustment for educational levels of people in different age groups:
https://metafact.io/factcheck_answers/2355
Based on Kaufman's research, the average full scale IQ for people 20 to 44 is about 100. Then it drops as follows:
45 to 54 years old - 97
55 to 64 years old - 94
65 to 69 years old - 90
70 to 74 years old - 86
75 years and older - 79
So lets assume Joe Biden is of average intelligence. He graduated in the bottom part of his classes at the University of Delaware and Syracuse University Law School. Some believe he wouldn't have graduated from either if he weren't a skilled plagiarizer. Throughout his career he's seemed somewhat mentally challenged from time to time.
I think "average" is reasonable.
Biden is 81 years old. So let's go with the "full scale" IQ of 79 above, for people 75 and older.
Here's an IQ percentile converter,
https://www.omnicalculator.com/health/iq-percentile
If I plug in 79, I get 8%. In other words, if Biden is of average intelligence for a person who's 75+ years old, he'd be pretty close to the bottom, in the 8% percentile, for people 20 to 44 years old.
I plugged that into a couple of other converters and got about the same number.
Think about it another way. Human Rights Watch says "to be diagnosed as having mental retardation, a person must have an IQ below 70-75":
https://www.hrw.org/reports/2001/ustat/ustat0301-01.htm
If Biden's full scale IQ is 79, four points higher than the upper end of the 70-75 range, we have a president who'd be pretty close to a retard if he were in the 20 to 44 age group. How much sense does that make?
Trump's not much better. If elected next year, he'll be 82 when his term expires.
We need new blood! Yes, Ronald Reagan did a great job. But he was "only" 77 when he left office.