The USATF Masters program is age group competition for track & field, race walking, long distance running, and cross country..
In track and field, masters divisons begin at age 30, while in all other disciplines, masters scoring begins at age 40.
Competition results are generally recorded in five year age groups (though sometimes in 10 year groups).  Competitions may be combined in track and field if numbers are smal in an event,  while masters results usually come out of full all-particiapant distance races. 

Abbreviations are gender and first year of an age group, so, M40 includes men age 40-44, F55 is women age 55-59, and so on.    Age in a competition is determined on day of event - or age on the first day of a multi-day track and field meet. Age is not the same for the full calendar year as in youth events.  
To "level the playing field", an Age Grading system has been developed to compare masters performances by age to open level record marks in each event.  Any age graded score of over 90% is considered World Class, and a number of USATF Masters Championships will award their age 40+ prize money on the age graded results. 
Many masters competitions are conducted concurrently with open championships. You can find compeition schedules under the various Sport links (Track and Field, Road Running, Race Walking, Cross Coutry, Mountain/Ultra/Trail).