Masters swimming has gained in popularity greatly in the last few years in the UK and worldwide. Generally it is for over 25s, who are no longer competing at top flight level (altho’ some Masters swimmers can and do compete in Open swim meets). Competitions are split into age-groups, with 25-29 being Cat A, 30-34 Cat B, etc. Some meets have a Cat X, which caters for 19-24 year olds.
Masters Swimming Clubs vary greatly in standard, from those that are purely recreational to those that are very competitive and have national level swimmers - the latter tend to be attached to larger swimming clubs that have extensive age-group squads. It is not unusual for Masters Squads to have a good relation with Tri-Clubs, and some have joint training sessions.
More information on Masters Swimming in the UK is avaiable from the ASA website: http://www.britishswimming.org