You are never too old to play in the sand, especially when it’s magic sand with science included.
As you know by now, magic isn’t really magic, there’s usually a scientific explanation behind what’s happening. Magic Sand is coated with a substance like Scotchgard that makes it hydrophobic or water-fearing. When placed in water, it repels it and stays dry. The sand is water repellant.
You can build castles and structures underwater and then pour the water off when you are finished. The sand will be dry.
For more on Magic Sand and where to buy it, read the experiment.