If an applicant states their age is 30 on an application for life insurance, when they were actually 37, what is the insurer likely to do?

It should be kept in mind that when filling forms for any kind of purpose, a person should never lie. In some cases, it might be considered illegal and against the law to fill the personal information wrong in some kind of forms.

In the scenario discusses in the question, the best the insurer can do is to pay the death benefit based on the applicant's actual age. As the age in the form is wrong, hence the payment should not be made according to that date.