The best time for a 20-year-old to get a flu shot is generally during September or October, ideally by the end of October. This timing helps ensure protection throughout the peak flu season. Everyone aged 6 months and older is recommended to get an annual flu vaccine, and at age 20, there is no preferred specific type of flu vaccine over another. The nasal spray vaccine is also an option if there are no contraindications. Early vaccination in July or August is usually not recommended unless the person may not return later for vaccination or is pregnant in the third trimester. The key is to get vaccinated annually with an age-appropriate flu vaccine by the start or early peak of the flu season for the best protection.