The bees don't start aging until they become field bees. Nurse bees that have never done field service are the bees that last through the winter, 3 to 6 months of time, depending on where you are at. So, all the field bees from a package will be gone in 45 days, but the nurse bees that were there when installed will then be the field bees 45 days later, hopefully with new brood, or close to a second round of brood, emerging as well by that time.