Among the challenges with (age) ratio IQs is the non-linear nature of development, which not only takes place at different paces at different ages, but also isn't uniformly positive. What we know about aspects of cognitive development is that certain elements (e.g., working memory, fluid reasoning) increase in absolute terms until about age 23 (or so), and then begin to decline, with a particularly notable drop-off at about 30 and then again about 40, followed by more gradual decline tracking senescence. At the same time, other aspects of cognition (e.g., crystallized intelligence/acquired knowledge & skills, wisdom/ability to integrate multiple experiences over time into one's thinking and decision-making) continue increasing over the lifetime (absent disease processes, such as dementia).

The kind of cognition that both the SBLM and contemporary instruments assess is heavily weighted toward the aspects that begin to decline after early adulthood (since these instruments were all designed to align with academic planning of some sort). (Vocabulary and Information-type subtests do draw on crystallized intelligence.) So while there is certainly something to be said for a measure that gives a better idea of "cognitive age", the SBLM wasn't really that instrument (although admittedly better at that than most of what we currently use). This is why I typically suggest using academic placement tests in the actual curriculum being used by the school in question for academic acceleration decisions. Why use proxies or predictors for appropriate placement instead of the actual tasks designed for placement in that curriculum?
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...pronounced like the long vowel and first letter of the alphabet...