you can exist purely in the present moment, I know I did when I was younger, and I'm sure many young people still do.
When we begin to perceive our situation in life, when we begin to think about how it is, how it was, how it could be different, better, or worse, when this happens, we cease to live in the present.
When the future becomes a thing of concern for the individual and he is no longer bound in the present, is when one sees one’s life for what it is, and understands how it could be different. When we begin to judge our life and our lot, that judgement brings about a change.
We live in the bubble that is the present moment, without any sense of doubt about our actions and lives, free from any sense of urgency about our limited lives. As we develop and as our situation begins to change, we leave school, our close friends move across the country to various universities etc, we start to think about the future more than before.
We develop a sense of finitude about our lives, and begin to understand that things do not last forever. We develop a greater sense of time as we age, and begin to feel a sense of urgency that we previously did not have.