Believe minium age is 10, but my kids are now adults so haven't paid much attention to that minor detail.
You need to sign up in advance, and then everyone will meet at a designated area to check in prior to departing the ship in a group. You'll see serious runners, social joggers, and walkers doing the 5K. CMs will be stationed at several water points to provide water, and keep track of the participants. They will have a bin that you can store any day bags in, but a lot of people just return to the ship to clean up a bit and change from running gear to swimsuits, etc. I've done it both ways and prefer returning to the ship.