The GHQ is ok, but probably better is the Geriatric Mental State, which has a wider posible diagnosis and a community shorter version is available; moreover, it can be automatically corrected by means of the AGECAT computer program. The main inconvenience comes from its length, but is well known and widely used (what is OK for future comparisons)
Excellent tools have already been suggested but I’d also suggest the Mini Mental State Examination, which tests memory and cognitive skills, and the four-item Geriatric Depression Scale as other tools may miss dementia and depression on the elderly that is also very important. Here are more information about various assessment tools for the elderly: