The two organisations said the arrangement would help The Silver Line, which was founded by the presenter Dame Esther Rantzen and runs a 24/7 support and companionship helpline for older people, to help more older people across the country.
The Silver Line’s helpline receives more than 10,000 calls a week and its befriending services have 2,000 beneficiaries.
According to the Charity Commission website, The Silver Line had an income of £4.7m and spent £4.5m in the year to 31 March 2018.
The commission’s website says the charity employs 195 people and has about 4,500 volunteers.
In contrast, Age UK had an income of £147.4m and spent £149m in the same financial year, as well as employing 1,503 staff and having 7,500 volunteers.
It is not yet know whether the agreement between the two charities will lead to any redundancies or what the impact is on The Silver Line’s offices and leadership.
Steph Harland, chief executive of Age UK, said: "We are very excited about what this partnership means for older people.
"By joining forces with The Silver Line we’ll strengthen our complementary services with the aim of helping more older people while making every penny of funding stretch as far as we can."
Rantzen said: "Meeting the escalating demand and finding the funds to answer every call has been a constant and exhausting challenge, and joining forces with Age UK is an exciting new chapter in the life of The Silver Line.
"It follows a very similar story to when we joined Childline with the NSPCC in 2005. Looking back, that was the best decision we ever made, and that partnership enabled Childline to grow and develop in ways that would have been impossible had the charity remained on its own."