Realising that all the things you were passionate about and wanted to do, will never happen because you are already over the hill and on the way back down the other side again. When you are joined by people in your work team and you are old enough to be their parent. They have sooo much energy, ideas and enthusiasm and you realise that used to be you, now you drag yourself in in the morning and stagger out at the end of each day.
Sorry to be a downer. Getting old sucks and then you die a long slow painful death. Oh what joy there is ahead to look forward to π