It's not a final thing you do, like graduation. It's foundational. If you get it right, marriage means having a co-founder for each other's life.
Most accept the plusses but then seem to think "Well I shouldn't aim for that any time soon, or at all when I'm young, since if it goes wrong..."
So they put it off for a decade plus
(if not, you may be bound for a nihilistic status trap. @webdevMason has good thoughts here)
Think hard about "doing" things: creating, taking risks, affecting the world and others in some way.
But 5 more years before kids also means subtracting 5 years of your life with kids and grand-kids.
(And obviously the younger you are, the easier it is to deal with loss of sleep, keeping up with kid energy, etc.)