My answer is as a rule of thumb, anything under 23 is too young.
Part of that is having seen a hell of a lot of teen marriages self-destruct before the first or second anniversary. Of course, that was also while I was in the Navy, and being on a ship that spent up to 11 months out of the year at sea was Hell on family life, too.
There are exceptions to that number, of course, some people should be much older. Others may be ready at 18. Just saying I cringe anytime I hear of someone marrying younger than that.
I just got married at 22. He’s 25. I’m not entirely comfortable with how young we were/are in taking this step, but our situation didn’t allow us any gradual steps towards marriage. I’m Canadian, he’s American. So, while visiting back and forth for four years while we were both in school suited us just fine, once we’d both graduated and started working, we had to face either seeing eachother alot less as time off from work was much more rare than time off from university (which was exactly opposite the direction we wanted to go in with our relationship), or going all out and getting married so that we could live together.
Left to our own devices, we’d have just moved in together and maybe made it ‘legal’ in several years. I would have liked to live on my own for a while after university before living as a partner. But we’re happy, neither of us had delusions of white weddings and fairy tale marriages, and we’re certainly in no rush to do anything more than take care of eachother. We wanted the same relationship we’d always had, except with us together all the time, instead of for a week or two every second month, and that’s pretty much what we’ve got.
So…while I suppose I don’t really consider 22 to be an ideal age for marriage, I couldn’t consider “I love you and I want to be with you, but I met you too soon so we’ll just have to drift apart” to be a viable option either.
I was surprised at how little hassle we got over it, really. I can count the snarky comments on one hand, and none of those were to my face.