It’s tremendous for adults, too–for pretty much the same reasons. If your primary pursuit is running or triathlons, swimming is a tremendous complementary workout. Particularly in the offseason when you’re not training for a particular event.
Now swimming isn’t perfect. Some have said it actually increases your appetite, therefore it’s not great for trying to lose weight. Whatever. If you can swim but haven’t done so in a while, find a pool and re-introduce yourself to the greatest cardio-vascular exercise there is.
And if you can’t swim, learn–if for no other reason than to be able to more safely enjoy the beach.