What do you think cognitive decline means? IQ tests are also called cognitive assessments and the accurate ones (not the ones you take online) involve a lot of different aspects of assessing how your brain is working. Many psychologists don't like the term IQ because it's too basic of a term...which is why many papers studying the subject do not include it.

http://pictonpsychology.com.au/home-2/assessments/cognitive-...