At what age were the Brontosaurus' young able to look after themselves?

Apatosaurus, better known as Brontosaurus, is a member of the sauropods, an instantly recognisable group of dinosaurs that had long necks terminating with small heads, long tails, barrel-shaped torsos and trunk-like legs. Brontosaurus, like other sauropods and all dinosaurs, laid eggs, whereas nearly all mammals give birth to live young. This would have been a major advantage in hazardous times and environment, when they would have been able to have many clutches of eggs, and lots of young every year. Clutches contained up to eight eggs, each approximately the size of an ostrich egg. Thus far, no evidence of parental care of Brontosaurus nesting sites has been found. It is likely that eggs were laid and abandoned, allowing the adult additional time to produce lots of offspring. Some of the Sauropod's eggs have even been found in a linear pattern and not in clusters, suggesting that the eggs were laid as the adult was moving along.