WASHINGTON — Baby boomers started driving at a young age and became more mobile than any generation before or since. They practically invented the two-car family and escalated traffic congestion when women began commuting to work. Now, 8,000 of them are turning 65 every day, and those retirements could once again reshape the nation’s transportation.

How long those 74 million people born between 1946 and 1964 continue to work, whether they choose to live in the suburbs after their children leave home, or whether they flock to city neighborhoods where they are less likely to need a car will have important ramifications for all Americans.