Complex sentence: This practice of base-following has continued up to the present and today at least 12 million Americans aged between 18 to 80 grew up moving from base to base all over the United States, and around the world.
Split into simpler sentences: This practice of base-following has continued up to the present. Today at least 12 million American (currently living) Military Brats (ages 18 to 80) grew up moving from base to base all over the United States and the World.