There are changes between when you’re 19 and then your mid 20s.
ASIDE from discussing financial junk, kids, religion, parents…
There are two main things to consider, AFTER all that other stuff.
1) The social effect
When you’re 19, you haven’t really experienced people or life as “adults”, and living in a more adult world can really change your personality/perspective/believes/wants.
Being around other established adults can help you develop different preferences, show you different personalities that you never experienced before (because they were all stupid teenagers, or because you just weren’t in the right situation, like a real work environment or whatever), or just see how someone completely different lives and makes do.
2) (main one) Biology changes things
You might not want kids now, but REALLY want them later. Or maybe you’re very peppy/excitable now, and when you’re 25 you’ll become more mellow/laid back. BUT MOSTLY…. your brain isn’t completely finished at 19 or 20 or even 21. It doesn’t actually stop developing until your mid 20s, generally around 25ish. This is obviously important because your brain is your personality (not just chemically and physically, but also how it deals/reacts with social environments, as stated above. You’ll get used to people overall in an “adult” world, and you develop expectations, preferences, annoyances, different opinions and values, etc).
This is one reason it’s so dangerous to get married at 20ish, even if you’re 100% mature in your decisions, and have talked about all the important things, if you get along perfectly… everything. You’ll be changing for a few more years, and you might end up different enough (or between the two of you, you’ll both be much different from each other) that it doesn’t make for a good marriage anymore.
But nobody ever mentions these things. They tell you that you need to be financially stable (which is an important suggestion), you need to discuss family and future, and you might need outside approval for a different confirming perspective (parents or whatever) and they might say “you’ll change a lot” but if you’re already mature and have discussed all those things, you question what could change? Someone saying “you’ll change” vaguely really isn’t very convincing, but based on the science, you actually will. But really, none of that stuff ultimately matters when you’re 19, because in 5 years there’s a really good chance it won’t all be the same, through little control of your own.
I’d probably wait until you’re about 23 at the earliest. 24 or 25 would be better. Plus hopefully one/both of you will be done with college, be able to get a house, and be financially stable. Always a plus. 😛