Consider drafting one once you turn 18 and are self-sufficient, said certified financial planner Catherine Seeber, principal for Wescott Financial Advisory Group in Philadelphia. It's worth having a will even if you don't have a complex financial situation or much to bequeath. Without one, state intestacy laws determine who gets your assets, which may not be whom you'd prefer. Basic wills can often be done very easily using software, said Sandy. Ask your lawyer about including a digital asset provision to grant your executor online access to bank accounts, email and other accounts, said Seeber. Some entities may balk at allowing that, otherwise.
Letter of instruction
Typically, this document goes along with your will, providing details like your preferred funeral arrangements. But planners suggest including more details, particularly log-in information for your online accounts and locations for important physical documents like insurance policies and deeds for a home. "In the old days, you used to be able to go through somebody's office drawer and piece these things together," said Seeber. "You can't do that anymore because everybody is doing it electronically."
Such a list ensures that online-only accounts, like PayPal, don't go unnoticed. It also ensures your heirs can manage accounts in the way you might wish, said Paré. Some companies won't allow access otherwise, and policies can vary widely. For example, terms for Yahoo accounts say they are nontransferrable: "Upon receipt of a copy of a death certificate, your account may be terminated and all contents therein permanently deleted."
Read MoreWhy your digital library will die right along with you
"People typically focus on having wills in place, but if your assets are beneficiary-type assets, it's not as important," said certified financial planner Becky Krieger, senior director for wealth management teams at Accredited Investors in Edina, Minnesota. Those include life insurance policies, many kinds of retirement plans and annuity contracts, among others. Listing a beneficiary allows the assets to transfer automatically to that person when you die. (Otherwise, the default beneficiary is your estate, sending the assets through probate, which can be a lengthy and expensive process.) Make sure to keep designations up to date, she said. They trump even your will, meaning those life insurance proceeds will still go to your ex-spouse so long as he or she is listed, even if you'd prefer otherwise.
Having a policy in place becomes important once your passing would cause financial harm to others. "Typically, we think about that for people when they get married and buy a house," said Krieger. Think of it this way: If you died, you wouldn't want your spouse to struggle to pay a jointly obtained mortgage that was suddenly less affordable without your income. Having kids adds to the need for insurance. Single parents and stay-at-home parents should have insurance. How much you need varies; financial advisors often recommend having enough to zero out debts and replace your missing income for your heirs.
Planners don't usually recommend that young, single people have life insurance. But it can make sense in select situations, said Sandy. Parents might consider taking out a small policy on an adult child for whom they have co-signed student loans. "You do leave that debt behind," she said. "Life insurance closes your obligations."
Read MoreHow much life insurance you really need