I think it starts around the time you become incredibly self-aware. All you see is you and your problems and you also think you can see how everyone else can see you.
And with that self-awareness comes your awareness of everyone else around you. You compare and barter with yourself deciding what pieces of other people you want to pick up and what pieces you want to leave behind.
Eventually, you get to an age where you start saying things to yourself, mostly in your head, but sometimes out loud about how "you would do it."
It may not seem like you're planning, but you are. You subconsciously start to plan your life. How you'll "do" college, how you'll "do" relationships and friendships. You make mental notes of things you will definitely buy when "that time" comes. How you'll decorate or build when you have "your home." Everyone does this, for a certain amount of time in their lives. I'm sure some people do it until the day they die.
But when your plans are rejected by something you don't control or can't adjust, your mind starts to shift.
All of those times you would have automatically said to yourself "When I..." and filled in the blank with your subconscious plans, instead of filling in the blank you stop yourself. Because what you thought would never be your reality, is currently your reality. The possibility doors close one at a time and as those doors close the light that once lit your perceived future progressively dims.