I got married (for the first time) at 18.
I appreciate that my family were trying very hard to respect my autonomy as an adult. I think there was probably a fair old whack of fear that if they voiced objections it would place a wedge, and make me dig my toes in harder. We did live together for two years first. But... that was no immunisation from too much, too young. I think part of me was pushing the boat out to test how far into deep water I could go before they broke the "your life, you're an adult,new respect that" line. In a sort of overgrown toddler way, I think I was testing boundaries. Perhaps in the hope of finding some. I got in too deep and had to sink or swim as a result.
People, despite similar sounding circumstance, are so different, with wildly diverse motivations, and different odds on a good outcomes. So even having had experience of a very early marriage myself, I'm hard pressed to have any advice as to what you should do, or say... becuase I could be so very, very wrong in my understanding of how it would play out in somebody else's real life.
But you evidently love her. And whatever happens that is going to be a vital part in how she manages her ups and downs. Be they related to her marriage, or not.
If it does all fall apart at the seams, you'll be there. Both she, and you, will get through it. My first marriage might as well have had a big red sign flashing "What a Fuck Up in Waiting!" flashing over it. But all the same, 30 years later, I don't regret it. I learned a lot about me (things like "perhaps rushing in where angels fear to tred is not life's best policy") and ultimately, despite the pain it caused me, him, the people who loved us... I wouldn't have just celebrated 20 years with DH or have our 15 year old son if that first marriage hadn't happened. And life is very very good now. So... it all came out in the wash.
At the same time cos I can imagine how hard it must be in your shoes. Looking at DS it seems unimaginable that over the next few years his growing autonomy will leave him free to make life changing decsions, and all I get to do is stand and watch, trying not to be too obvious about my arms opening in advance ready to catch him should he fall.
I am really not looking forward to that bit at all. Too much concern that will be the time when nemesis comes to bite my arse as payback for my young adulthood and all the grey hairs I gave everybody around me.