Position classes (cubmaster, Denleader, 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 camping seasons
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