Hello Wendy! Welcome to the Disney Parks Moms Panel.
Disney's Keys to the Kingdom Tour is a fantastic in-depth look at the inner workings of the "show" that makes the Endless Magic of the Walt Disney World Resort's flagship Theme Park, Magic Kingdom, a modern wonder.
Having experienced Disney's Keys to the Kingdom Tour, I can say that the age requirement of 16+ for the tour is due to the nature of the experience. The tour focuses on discussing "how things work" and the hidden details and story behind the pristine veneer.
Walt Disney World Resort is a place where kids are presented a polished show with magic and whimsy at every turn. Disney's Keys to the Kingdom Tour is more of a question and answer based tour and kids surrounded by Adults asking questions about sanitation systems and discussing historical building plans may not be as magical.
Disney's Backstage Magic Tour keeps things lively and constantly moving to maintain younger Guests attention throughout. It also contains more attraction experiences and a special Character Experience for Guests of all ages to enjoy the tour. The higher cost is based on the greater length for the tour and access to multiple locations (as opposed to only Magic Kingdom).
In my opinion, Disney's Keys to the Kingdom is an adult-centered educational experience, while Disney's Backstage Magic Tour is an interactive, immersive Family experience.
Walt Disney World Resort has a tour for every age and interest. You may contact a Tour Specialist by calling (407)939-8687 to make reservations or to find the "perfect fit" tour for you and your son.
Good Luck!
~Todd