Masters swimming is simply the name some people use for
adult swimming.
The term originated in the USA in the 1970s when some former
swimming champions organised a competition for adult swimmers.
Nowadays, Masters swimming refers to any kind of swimming for
adults, from casual fitness swimming to competitive swimming. You
don't have to be a former elite swimmer, you just need to enjoy
swimming.
To qualify as a Masters swimmer you need to be at least 25 years
of age. Some swimming clubs have Masters sections and a few
swimming clubs are dedicated to Masters alone. Find your local
club.