They are lagging well behind the 68% of those aged 55-plus with wills, according to new research from mutual insurer Royal London.
Although younger adults are less likely to have a will, they are more likely to have children who depend on them.
Nearly a third of those aged 18-34 have children who depend on them, compared to only 14% of those older than 55.
Not having a will could create unexpected complications, such as assets being inherited by another family member.
Of those that have a will, nearly a third of younger adults were prompted by having children, a fifth when buying a property, and 29% said it was just something they know they needed to do.
For young adults that don't have a will, 40% feel they are too young to worry about it, and a third have just not got round to doing it yet.
November is Will Aid month, when participating solicitors volunteer their time and to write a basic will and ask clients to make a donation to Will Aid's charity partners.
Sarah Pennells, a consumer finance specialist at Royal London, said:
"Writing a will is not just something you should do when you hit middle age.
"It's particularly important to consider once you own a home, have children, start a business or have savings and investments.
"Creating a will can be a task that never makes it to the top of the 'to-do' list, particularly for younger adults. But 86% of people found writing a will easy, and it could prevent unnecessary complications at an already-difficult time for loved ones.
"A solicitor will look at your estate and arrangements for any children to make sure that what you want to happen will happen, when you die."