Children who never leave home cost their parents an average of £72 a week each.
A report into the phenomenon of 'boomerang kids' (young adults still living at home) found that a quarter of all over-50s have adult children living at home – many living for free!
They don't contribute to the mortgage, rent or food or fuel bills, thus costing their mums and dads around £3,750 a year.
Official estimates say there are well over three million young people in their 20s and 30s who still live with their parents, and that one in five young men are still in the old family home at the age of 28.
Many have returned from university and prefer to live cheaply with their parents than face the daunting costs of renting or buying their own home.
Dominic Grinstead, director of insurance company MetLife, which carried out the research, said: "Helping out family is a powerful motivation for parents and it is understandable that many over-50s are happy to allow adult children to live at home for free.
"The financial pressures on young adults from the need to save for a house deposit while finding a job and in many cases paying off university debts make it almost inevitable that millions of young adults have to live in their family home.
"But parents need to think carefully about how they combine helping family with ensuring they are maximising their own retirement saving."
The survey of more than 2,500 people found that one in 10 people over 50 allow adult children to stay at home without making any contribution to their rent or keep.
They make up more than four out of 10 of the couples who have boomerang children.
The adult children who do offer help to their parents on average pay £51 a week towards the bills.