If the essence of youth is a boundless view of time, space and opportunity allowing for the wildest dreams in life to be made, then it should follow that dreams are essential for youth culture. And I believe this to be true. Let us have a more detailed look to see if this is really the case.
A recurrent theme in the culture of being young is simply that you have the ability to do pretty much everything you want to. From Billie Piper's Because We Want To in 1998 to Katy Perry's Roar in 2013, the music of youth celebrates this idea very well. Movies celebrate this idea as well. Movies from Spiderman to Transformers are all about unlikely people becoming heroes, doing previously unimaginable great things.
Another manifestation of this is in its talk about relationships. A lot of music and movies have that theme where somebody is able to get the unlikely dream boyfriend or girlfriend, either as the main theme or an important sub-plot.
Yet another manifestation is in those movies about having a big adventure, the most popular settings being a road trip with friends or an overseas adventure. There are too many movies here to name, really.
As you can see, the spirit of dreaming big is not just present as a part of being young, it is interwoven into every aspect of life, and appears in different forms in different parts of culture or life. It is omnipresent.
Put it simply: if you want to regain the spirit of youth, the spirit of dreaming big must be omnipresent in your life and your culture.
Some Related Musical Inspiration: