How old do you have to be to get into a green day concert?

It depends on the venue. Many venues are all-ages, while concerts at bars and other venues that serve alcohol may be 18+ or 21+. These age restrictions will vary by country depending on alcohol laws.

The age restrictions for any specific concert are usually listed on the ticket-selling website (Ticketmaster, LiveNation, etc.). You can also call the venue or look on their website.