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.