They usually only live for one year. They are born during the winter and they grow into large adults by the summer, then they have their babies and they die before the next winter. Sometimes if we have a particularly warm winter they don’t die but they carry on growing. It was for this reason that we had so many really large Lion’s Mane jellyfish washing up on our beaches this summer. It was because we had a warmer than average winter so they didn’t die but kept growing to enormous sizes and were nearly 2 years old before they died.