Why do you think bones don't show growth marks before adulthood?
Nope! The answer is:
Young sauropods grew too fast for these lines to form.
Young sauropods grew too slow for these lines to form.
Young sauropods do not eat enough for these lines to form.
Young sauropods grew very rapidly. Some gained up to 5,300 pounds (1,600 kg) a year! Their growth usually slowed around age 30, when they reached adulthood.
I can estimate a sauropod’s age by examining thin slices of femur under a microscope. Each slice is about half the thickness of a single strand of hair! See the lines? Each line is a “growth mark,” showing one year of adulthood. The space between the lines shows how much it grew in that year. So like the rings of a tree, they can be counted to reveal the age of the dinosaur. They also tell us how the animal grew.