Annual influenza vaccination is recommended for nearly everyone from 6 months of age. It matters most for adults over 65, young children, pregnant people, and anyone with chronic heart, lung, kidney, liver, neurological conditions, diabetes, or a weakened immune system. Healthcare workers and carers of high-risk people are also strongly encouraged to get it.
The flu virus changes each year, so the vaccine is reformulated annually and immunity wanes over months. That is why a single past shot does not provide lasting protection. The best time in temperate regions is early autumn, before the season peaks, though getting it later is still worthwhile.
The main reasons to delay or avoid it are a severe allergic reaction to a previous dose or a vaccine component, or being moderately or severely unwell on the day. A mild cold is not a reason to skip it. Talk to a doctor or pharmacist if you have an egg allergy or a history of Guillain-Barre syndrome, as suitable options or precautions exist.