The organisations registered the new charity under the legal name of Age UK when the merger happened in April, pending a further rebrand. They hired brand consultancy Corporate Edge in January to come up with a new identity.
But the organisation has announced that Age UK will be the name it will use – although not until spring 2010, when the rebrand will be officially launched.
Regional organisations will use the names Age Cymru, Age Scotland and Age NI.
A spokeswoman for the merged charity said the delay was to give partner organisations, particularly local Age Concerns, time to prepare for the launch.
Tom Wright, chief executive of the charity, said the new name "clearly reflects who we represent and what we do.
"The research we have carried out over the past eight months has not only helped us choose the right name, but given us the chance to hear how we can best represent and support people in later in life," he said.
"This has been crucial in helping us develop a shared vision for the new charity so we can deliver real improvements to the lives of older people."