Answer:
"Too soon" would be before you know you could live, and have lived, in the same space as them for months on end. If you have handled cohabitation for a year, then marriage is not a bad idea.
Loving them with everything you have is also a good start, as is having something to build the marriage itself from.
18-22. . . You are still figuring things out even when you are 24-25.
The heart doesn't not know what it wants sometimes at a young age but if the persons are just meant to be, 24-31 would be anything but too soon and anything before is soon or higher risked to be regretted.