They can retire as early as 34, 35 years old or as late as 41-44 years old. I think they commonly choose to retire around 37-40.
34, 35??? The youngest I've seen was 39.
The first driver I ever hired to my team retired at 36!
Less than a year ago I am pretty sure I saw a driver who was retiring/retired at age 34. Can't remember if they were employed or not.