This statement identifies a weakness. This statement provides a new reason why old people sleep less than young people. The reason behind old people sleeping less is not because of their age but because of their impairment in memory.
2 – Wrong
Although this may be a factual statement, it does not provide a weakness in the argument made by the paragraph.
3 – Correct
This statement says that ageing is not the only reason why old people are unable to sleep well. It identifies a weakness and states that changes in brain due to ageing causes deterioration in memory and sleep patterns.