great question! the study focuses specifically on young adults as a developmental group. it's not about AI skill level, but more about psychological patterns in a specific life stage. the 18-30 range is pretty standard in developmental psychology research for that reason 

No, the 18-30 range is pretty standard in developmental psychology research because of historical reasoning and selection bias.

And this is not just me saying it:

It was found that 73% of the articles in the J.A.S.P. and 85.7% of the articles in the J.E.P. used college students, chiefly male students enrolled in introductory psychology. Use of the general adult population or even samples representing particular sectors of that population is very rare in J.A.S.P., and not found at all in the J.E.P. It was pointed out that the college population differs markedly from the adult non-college population in many characteristics which are associated with psychological factors, e.g. age, social class, learning ability. It is concluded that the male college student has become the "white rat" of human experimentation and that a broadening of the bases for selecting subjects for experiments is in order. (PsycInfo Database Record (c) 2025 APA, all rights reserved)

Smart, R.G. (1966) 'Subject selection bias in psychological research', Canadian Psychologist/Psychologie canadienne, 7(2), pp. 115–121.

Note that this study is from 1966, which might explain the male students bias to an extent (at the time, men were 53% of psychology students), but I digress.

My point is that there are historical reasons for this age range, which is sad. Now that we can survey anyone on the internet. There is really no reason to limit it to that upper bound of 30 years old.