In your 20s (and once again this is more about where you are in life mentally and less about your physical age, but age averaging just makes things easier to talk about) you’re so much more likely to put on a facade or not even know who you really are. In your 30s and 40s and beyond you tend to be more secure in who you are and present it more forwardly. A year and a bit don’t seem long when you’re trying to figure out if they’ve presented the real them and if they’re going to like the real you.
But it seems a lot longer when you were both forward with your desires and personalities from the very start. On top of the hole where you are in life, the thing is a factor, there’s also just the factor of different people being different people. I know some adults who wouldn’t get married before like 5 years of being together, and I know some adults who never want to get married at all.