Chronological age and crystallized intelligence of people with intellectual disability.

The team looked at 6- to young learners with intellectual disability.

They gave standard IQ tests that split intelligence into two parts: fluid (problem-solving) and crystallized (stored facts and words).

Then they asked how much each part, plus plain age, predicts a child’s fact-and-word store.

Age alone explained 21 % of the differences in stored knowledge.

Fluid reasoning explained 43 %.

Bottom line: kids with ID keep adding new facts and words as they get older, even if their problem-solving stays flat.

Boutros et al. (2011) saw no age jump in self-regulation during problem-solving, but they only watched short tasks, not years of growth.

Vakil et al. (2011) found adults with ID still use simpler eye-movement tricks on analogy tests—showing fluid skills may plateau, which lines up with B et al.’s lower fluid numbers.

Kocher et al. (2015) tracked preschoolers with cerebral palsy and saw younger kids lose participation fastest; B et al. flip that script—older kids with ID keep gaining knowledge, not losing it.

Together the papers say: expect flat problem-solving but steady vocabulary growth; keep teaching words even when “IQ” looks stuck.

Keep stacking language and fact lessons across childhood and high school.

Don’t quit on vocabulary goals at 12 because the score seems frozen—crystallized intelligence keeps climbing.

Track both kinds of smarts: use fluid tests to pick problem-solving goals, use age-plus-language probes to set vocabulary goals.