On the one hand, we keep changing (maturing) until we die.
Nevertheless, there are some milestones that are very real.
I see this with little children, for example --
I have a 5 year old, a 3.5 year old, and a 1.75 year old. They're all relatively close in age, and any 2 of them can play together.
I give my 5 year old some scissors, and the 3.5 year old wants to play with them to -- "I guess..." but the 1.75 year old sometimes wants to do what they're doing. Much of the time, the two younger ones play along together on the same level.
But at some point, you have to draw the line "I'm not letting the one-and-a-half-year-old play with scissors!"
The difference between them is gradual, the lines are blurry, but the differences become very concrete and real in certain cases.
Just like no one would say the 1.5 year old should play with scissors like a 5 year old, no one would say an 18 year old is as "mature" as a 24 year old.
I realize there is no hard-and-fast standard of maturity for something like marriage, and that 18-year-olds vary in their maturity.
It's common sense that a 10-year difference doesn't mean as much if SHE is 35 and HE is 45, compared with her being 15 and him being 25. Why?
Obviously rapid change slows down once a certain level of maturity is reached.
Let's put it this way -- if you go to a 10 year High School reunion, everyone is going to be drastically different, especially those who didn't come to the re-union because they are a Catholic priest, a monk in Tibet, etc. the real unusual paths won't be able to make it to the reunion!
But I bet there is much less difference between the 10 year and the 20 year reunion. For the same reason I outlined above.