The new chief executive, who will be paid £150,000 a year, is expected to take up the post at the start of 2009. He or she will spend several months appointing a senior management team and preparing for the launch of the merged organisation.
Gordon Lishman, director general of Age Concern, and Michael Lake, director general of Help the Aged, will both stand down when the new charity is launched in the spring.
The merged organisation, which the age charities are referring to as 'NewCharity UK', will work predominantly in England. Three separately constituted sister charities that share the same name, brand and aims will be set up in Scotland, Wales and Northern Ireland.
Former National Lottery Charities Board chief executive Richard Buxton has been appointed programme director with responsibilty for leading the development and delivery of the merger programme and the creation of the new organisation.
Buxton, who oversaw the merger of the NLCB and also set up the Legal Services Commission, is working with a group of staff seconded from the two charities at a programme management office, which has been set up at Help the Aged's London base.
The merger will be the largest since the Cancer Research Campaign and Imperial Cancer Research fused in 2002 to form Cancer Research UK.