Unmarried gay couples, many of whom are childless, should understand that without legal documents, the partner of an ailing person may not be given a say in health care decisions. And the partner of a deceased person may not be allowed to stay in the couple’s house or to gain access to retirement funds or many of the benefits that a traditional married couple would have, said David M. Goldman, an estate planning lawyer in Jacksonville, Fla.

Don’t forget about pets, who can be almost like children to their owners, said Hyman Darling, an elder law lawyer in Springfield, Mass. A pet trust ensures that they will be cared for. Although Ms. Sommers will almost certainly outlive her dachshund, Romeo, she has arranged for her sister to take care of him, just in case.

More than one person can be in charge of your health care and finances, if you wish. But whom should you choose to serve as your agents?

Couples often choose their partners, and single people may designate siblings, but selecting people close to you in age carries a risk that they will die before you. If your first choice is around your age, you should appoint a backup who is younger, Ms. Kovacs Gruer said.

Nieces, nephews, friends, neighbors, clergy and fellow church members are all possible choices. Your agents should be people whom you trust and can keep updated if your wishes change. You may choose to reward someone after your death for serving as a trustee, but the person’s main motivation should be concern for your welfare, Ms. Kovacs Gruer said.

What if there is no one? Older people sometimes outlive their spouses, friends and families. It is an argument, though not the main one, for cultivating friends of all generations, she said.

Beware of people swooping in to perform these services for the wrong reasons. Mr. Darling said he tried to make sure his clients were not being coerced or unduly influenced by people with suspicious motives.