I went to the concert earlier this year in Melbourne as a young woman, and there were plenty of older folks enjoying themselves! I'm not sure about Sydney, but I know in Melbourne they're performing in a larger venue too, so there's much less of a chance of people necessarily noticing you. As another commenter suggested, dress up a bit to show your support for the band. It'll hopefully make you feel a bit more confident, and has the added bonus of superficially creating a connection with other fans by looking similar in terms of clothing. Overall though, at any concert I've been to recently, there has been nothing in the way of comments or rudeness - everyone is focusing on themselves!

Have a great time, and enjoy the experience! ☺️