Yeah, I'm another who doesn't think this makes much sense, as marriage doesn't suddenly mean you can't also focus on your career, education etc. Many people of 18+ will be in relationships - are you saying nobody of the age of 18 should be in a relationship as it will get in the way of their education, job, etc? Fair enough if so, I suppose, but I suspect very few people actually think that.
People do make choices and prioritise in life, we all have to. What to pick if things conflict, how to allocate time and attention. And some people - and, I think, especially young women - end up prioritising relationships over personal development at that age and it often doesn't serve them. They don't go away to uni because their bf/gf lives here. They don't take a job in another area because ditto or because their bf/gf doesn't like the idea. They don't travel because their OH can't afford it or they'd miss them too much or whatever, and yes I know not everyone gets the chance to travel and go away to uni etc.
That is not at all the same as saying no one can have a relationship at 18. But most 18yos will be disadvantaged by putting that relationship above other opportunities, which marriage often obliges you to do, indeed it's in large part a statement that you're willing to, because those 18yo relationships generally aren't particularly strong and generally don't last.
None of this stops two people who have truly found their soulmate who strengthens them and so on from staying together at 18 or indeed getting married, nobody can stop them if they're dead set on it. But why rush? You can still be each other's inspirational soulmate without a ring.