However, in order to buy the policy you probably need to be at least 18 years of age, or the owner and purchaser of the life insurance plan would probably need to be your parent or guardian, or some adult with an insurable interest in you, such as, your grandparent.
As far as buying life insurance as an adult, you may still be able to qualify for life insurance as old as age 78 or even older. It really depends on the life insurance company and the type and amount of life insurance you need. As well as, your health, age, and other underwriting factors the insurance company considers when you apply for life insurance, such as, your height-to-weight ratio, if you smoke or not, your driving record, etc.
Here's an article that reviews term life insurance for elderly people and explains how you can compare life insurance plans online and even find out if you qualify for life insurance without any health exam required.