If they're still rolling, they're still rolling. They were produced over a span of 7 years, and it might take just as long to wreck them, or even longer. The BR38 might be a good example. I can't find any dates, but I suspect they started wrecking them long before the final one(s) were retired in the 1970's. Other times, the process of retiring a locomotive class has to be put on hold because of unforeseen problems with their replacements.
The BR38 also suggests that the retirment date in Simutrans is the date for when the process of retirement is completed, not when it started.