But how highly functional are low-IQ people on average? Looking at only the most functional low-IQ people wouldn't give a good idea of what to expect in general.

My point was that different groups of people that might score low on IQ tests sometimes differ systematically from each other. In my last paragraph I suggest quite directly IQ 70 Black people are different from IQ 70 Whites and East Asians.

The 13-year-old's raw score would generally be lower, but their IQ should be about the same as the 18-year-old's; IQ tests are age-normed nowadays.

Well aware of this. Hence:

If the test got mislabelled as that of a 18 year old, what would be his estimated IQ? In abstract thinking he may not be that better than a 18 year old with a 70ish IQ but in many many other regards the test wouldn't do him justice by putting him in the same category.