Position classes (Cubmaster, Den leader, Committee Challenge, Scoutmaster specific, IOLS, BALOO, etc) never expire.  If you took the course in 1950 and have proof you took it (or the comparable course) then it is still valid today.  However, when there are major curriculum changes it is recommended that you take the course again:  recommended - but not required!

Required Training for Scoutmasters and Assistant Scoutmasters:  

Youth Protection, Scoutmaster Specific, and Introduction to Outdoor Leadership Skills

Required Training for Troop Committee Members:

Youth Protection and Troop Committee Challenge

Below is a list of courses that expire and how often they should be retaken to be valid:

Youth Protection-every two years 
National Camping School-good for five campingseasons 
Safe Swim Defense-every two years 
Paddle Craft Safety-every three years 
Aquatics Supervision/Swimming and Water Rescue-every three years 
BSA Lifeguard-every three years 
Visitation training-every two years 
Climbing Lead Instructor-every two years 
Climbing Instructor-every two years 
COPE/Climbing Inspector-every two years 
Safety Afloat-every two years 
Chain Saw Safety-every two years 
Trainer's EDGE-every three years 
Hazardous Weather-every two years 
Physical Wellness-every two years 
Climb On Safely-every two years 
Trek Safely-every two years