We did it on the June 4th cruise...and my son went, he's 11. They didn't ask about his age, but then...they know his age. Oh, but there were some kids there that were younger than DS.
And....at least on the Magic...do it now, before it goes to drydock. The catwalk that you stand on to see the bridge will be gone after drydock. They are expanding the excercise room at the spa, so you also won't be able to see the bridge from the treadmills.