All art is valuable.
Yes, this is true in a sense however, your 4 year old son’s finger painting on your refrigerator is slightly different than the Watteau hanging in the Louvre. I apologize that your child is not as great as the greatest artists that have lived. This does not take away from how special their work may be to you but famous, historic artists pieces are special to the world.
As an antique chairs worth increases over time, so does a paintings. That being said, mainly paintings by well-known artists such as Monet, Picasso, Pollock etc. are the ones that hold great value. Part of this is because works of art from the 1600’s are literal pieces of history. In such pieces, one can find the type of paints used during that time period based on pigments and markings. The pyramids are treasured because they were built years ago without modern technology. The paintings of the Renaissance time period did not use the same techniques because technology is advancing.
I believe that both modern art like Banksy and older art like The Mona Lisa are valuable. Though, they are valuable for different reasons, from a strictly academic viewpoint since I am not able to tell if a piece is sentimentally valuable to another person or not.
A piece such as Raphael's Portrait of Young Man is valuable due to it’s age. Like fine wines and cheeses, art gets more expensive with age. It is like a freeze frame into a specific time period. The paintings may represent the clothing and style, the jobs and architecture, the hierarchy of the times. On the other hand, a contemporary piece of art such as a Banksy piece is also incredibly valuable. A Banksy represents now and what is popular currently. Banksy is also a special example because their work is usually satirical/social activist pieces so they can be huge influencers to the public or statement pieces in general.
There are a few factors that impact the value of an art piece. Age is a huge one that I do not think will ever change. Social trends can alter the value of a piece. Abstract and very contemporary pieces are pretty “in” right now and thus their value may be perceived as higher than if a modern artist made a cubism piece. Pop culture trends and age are good predictors of the value of a work of art.
It is possible that one day your child's work may be worth millions but for right now, young adult pieces are not very “in.”