The Australian system is really flexible with entrance requirements, you don't need to have completed high school to get in. You can get in through a variety of alternative pathways, like professional experience, special entrance programmes, etc. Age is no particular barrier either, and it's pretty common to have older students.

It's recommended by a lot of people not to go straight into uni, especially if you're unsure what direction you want to go in. Spend a year or a few, work some hospitality jobs, and just enjoy yourself first, then go do uni in your 20s.