senescence - How is it that the WI-38 cell line isolated by Hayflick in 1962 is still very much around and not affected by the 'Hayflick Limit'?

WI-38 are the cell lines which led to the proposal of the Hayflick limit and are the classic example of cells which will only divide ~40 times. they are not exempt - they are the example that proved the rule.

Lots of cell lines are commonly available, but have to recultivated regularly from animal/tissue source. If they can be frozen, then the extraction of new cells doesn't happen so often without exceeding the limit of divisions. This allows the cell lines to be used widely for experiments despite the limitations on their division in culture.

Other cell lines like HeLa and other tumor cells or Stem Cells can, under the proper conditions, divide without limit and are easier to culture in large volumes, but are often inappropriate for a given research project.