When thou wast young - embracing the whole period of life to the verge of old age.
thou girdedst thyself, and walkedst whither thou wouldest - wast thine own master.
when ... old thou shalt stretch forth thine hands - to be bound for execution, though not necessarily meaning on a cross. There is no reason, however, to doubt the very early tradition that Peter’s death was by crucifixion.