The Welsh Senate of Older People will lobby the UK and Welsh Assembly Governments on matters such as age discrimination, housing and health.
The senate is being launched at the assembly in Cardiff Bay later.
It will also work to improve a wide range of services to the elderly in Wales.
The four organisations joining together are the National Old Age Pensioners Association of Wales, the National Pensioners Convention Wales, Older People's Advisory Group Cymru and Pensioners Forum Wales.
A spokesperson for the senate said: "The population of Wales is ageing at a faster rate than any other part of the UK, and in 20 years time one in three Welsh adults will be aged 65 or over.
"Older people's issues have become increasingly prominent in recent years and the creation of the senate is a significant milestone in the journey to unite older people the length and breadth of Wales."
Guest speakers at the launch will include First Minister Carwyn Jones, Welsh Secretary Cheryl Gillan, and the older people's commissioner for Wales, Ruth Marks.
Article from BBC News