Most insurance companies use your age at the nearest birthday, meaning within the first six months after your birthday your current or true age would be used; after six months your age would be considered nearer your next birthday, and the older age would be used. Therefore, if your age for insurance purposes will change in the next four weeks, you can lock in your lower age by paying the initial premium with your application, which will make your policy date the same as your application date. Or, you can sometimes choose not to pay your premium with the application and ask, if available, that your policy be backdated to take advantage of your lower age.
If your half-birthday is coming up in the next four weeks, you can still use your lower age.