If you rely on them for financial support, or you would suffer financially if they passed away, you may be able to buy life insurance on them.
This is called "Insurable Interest", and is required when buying life insurance on another person.
In addition, to insurable interest, you may need them to sign the application for coverage, and answer some health questions in order to get life insurance on them. However, some insurers may or may not require your grandparent's signature.
There are life insurance plans for seniors that offer life insurance coverage with no health exams required, just a simple online application you can fill out in a matter of minutes.
Here's how to buy life insurance for a parent or grandparent.