For several years my childhood bedroom was covered in clutter -- papers on the floor, books piled everywhere, scattered bags and craft materials, etc.

One day when I was about sixteen, with no prompting from my parents, I suddenly went "let me see if I can clean this up a bit". An hour and several trash bags later, my room was clean and stayed that way till I graduated.

Both my kids have messy rooms and have for years; I figure it's largely because I've bought them more stuff than we have room to store. Middle is not naturally organized and has an extremely hard time getting rid of things; Youngest has more inclination towards organization and decluttering but is still dealing with more stuff than the room has space for, plus there's only so long he can put up with cleaning his room until he gets bored and goes to play Roblox. Also, it's not like the adults are setting a good example!