The Commission on the Voluntary Sector & Ageing says that, by 2020, voluntary work by people over 65 will be worth more than £15bn a year to UK charities.
Its study, Age of Opportunity, says tens of thousands more baby boomers are reaching retirement age every year, with a skilled and demanding cohort of older people retiring or set to retire.
However, it says older people are under increasing pressure within their own families and to get involved local causes.
Lynne Berry OBE, chair of the Commission, said: "Our society will change dramatically in the next 20 years. This leaves the voluntary sector at a crossroads but the rewards, if they look ahead and plan now, are substantial.
"Our challenge is to make this ageing society work for all ages."