I'd want to live with someone for at least a year before we decided if we were going to get married, regardless of age, personally. You learn a lot by living with someone.
Obviously there are plenty of times where it works out fine, but there are plenty that don't - a third of marriages end in divorce, so reasonably high odds it won't work out.
You can't 100% know whether you'll last as a couple forever regardless of if you got married 2 weeks or 10 years in - but there ARE some things you can learn before to help you know if you're at least capable of getting through the basics together. How someone behaves when they're ill, you're ill, how they handle when something goes wrong in your home or big life stuff happens or you have a blazing row and still have to live together, how you coexist in a shared space, what they're REALLY like about sharing domestic stuff, how they handle (semi-)shared finances that come with living together (and any disagreements), how they are during a period of real stress (or they are with you when you're really stressed). DH and I got married after a very long time together but by that point we'd been through numerous family deaths, car accidents, serious illnesses, a house flooding, plenty of arguments, all sorts - now hopefully the average person doesn't have all of these things happen, but it meant that I felt as sure as it's possible to be (which is to say you can never been 100%, but as close as!) that I felt happy to tie myself to him financially and legally.
If you want children, these things are even more important, I think. How can you possibly know he's going to be someone who'll have your back if you have horrendous PND or birth injuries or an ill child (god forbid all of those things happened of course) - or even just the trenches of babyrearing - if you don't even know how he handles a period of stress otherwise??
Maybe I'm overly cautious but I wouldn't be agreeing to marry someone I didn't know these things about and I can't see how you'd know these things in that short a time period.