Getting to the Met involves a lot of things, only one of which is vocal skill. A random adult would not have a lifelong network of contacts, experience, acting skills, and a depth of knowledge of the repertoire that someone who grew up doing it would. And there are certainly arguments that brain plasticity and physiology plays a part in developing talented child musicians in a way that may not be replicable in adults.
But that aside, yes, an adult can be trained to a high standard of opera singing, sufficiently so to take on significant roles in commercial productions. Bearing in mind, of course, that we are talking here about 5-10 years worth of daily training - but it is possible. Just rare.