Why would you think that? Information is stored in the configuration of neurons, not in the neurons themselves. Neurons are constantly being replaced, and you don't wake up one morning not knowing how to bike because your "bicycle neuron" died.
Are they ? As far as I know, neurons last more or less from before birth to death. There is neurogenesis in the hippocampus but it doesn't replace existing neurons, and the new cells themselves last until death.