Len, I believe you are correct in that as a general rule 40 years old is the "Masters" threshold for distance running, however, there are at least a few race organizations which have their own definitions of the "Masters" classification; here is one such:
Reach the Beach Relay
Masters: All team members over the age of 30
Super Masters: All team members over the age of 40
Grand Masters: All team members over the age of 50
As for Track & Field, I think I read somewhere that they start at 40 and then have a separate age class every five years from there.
Individual events can, and often do, define various categories according to age. Many events give age group awards in 5-year or 10-year brackets. For distance running, I know USATF officially defines it as 40 and above, with no further age-group distinction. That is, everybody 40+ is a "Master". So you have to be 40+ to set a sanctioned Masters record, for instance. They recognize age-group records separately. I have seen some track events that start masters at 35, and I wasn't sure if that was official or not.