Depression among the elderly is three times as common as dementia and must be addressed in the Scottish Government's mental health strategy, Labour warned.
Alcohol abuse, psychosis and suicide rates among older people are also concerning, Dr Richard Simpson told MSPs.
Speaking during a Holyrood debate on mental health, the Labour MSP said good progress had been made in recent years, but he highlighted areas where further work is needed.
Dr Simpson told the chamber that public health minister Michael Matheson had not emphasised "the fact that depression is three times as common as dementia".
"Depression in the elderly is estimated to have a prevalence of around 13.4% and is predicted by the World Health Organisation to be the second highest health burden in the western world by 2020, and is strongly linked to disability," Dr Simpson said.
"And yet only 10-15% of these are treated, unlike depression in younger adults. The presentation to psychiatrists is less and the presentation and diagnosis by GPs is less."
He added: "Suicide rates in the elderly, unlike the excellent progress that has been made in younger adults, is almost completely unchanged over the 10-year period (from 2002). Psychosis is also increasingly common because of the fact that we have got many more people over 85, and that group are associated with greater numbers of psychosis."
Dr Simpson said addiction specialists had also reported concerns about the levels of alcohol consumption among the elderly, which is linked to mental health problems.
His comments came as Mr Matheson updated the chamber on the progress of Scotland's Mental Health Strategy and set out its aims for further improvements.
He said there had been a steady reduction in re-admissions to hospitals for mental health reasons and progress on suicide prevention while work would continue towards offering faster access to mental health services for children and young people, and increasing the pace of change across the whole of system to reduce variation in availability of mental health treatment.