When to buy life insurance depends more on your individual circumstances and your personality than on any hard-and-fast rules. There are good arguments on both sides of the issue.
Life insurance premiums increase dramatically as you get older. If you buy permanent life insurance while you're younger and healthy, you can take advantage of lower rates. Your premium costs may increase, but these increases are strictly regulated. Cash value life insurance also offers a type of "forced savings," and the policy accumulates tax-deferred over time. Buying while you are younger provides the added advantage of extra time in which the cash value can grow.
Buying life insurance while you are younger can be especially important if you are in a high-risk group for any disease or medical condition that might make you uninsurable. If you choose to buy insurance for this reason, ask about adding a guaranteed insurability rider to your policy. This rider guarantees you the right to purchase additional insurance at specified times, without having to provide proof of insurability. This way, if you get married, start a family, or take on other responsibilities, you can purchase the insurance you need without worrying about whether you will be denied coverage.