At 8 metres tall at the hip and 12-15 metres long, it is clear that they were just that size indeed. Fully grown adults would have had nothing to fear from predation. Juveniles would have stayed under the protection of their mothers for an extensive period of time, until expelled from her presence by force.
Males would battle each other for breeding rights, and Palaeontologists have analysed the skulls, and found that they were reinforced in the males specifically. Why? Well, the best guess is that they were reinforced in males to allow them to better cope with contests of strength, hitting each other in the sides with colossal strikes of the head and neck. Indricotheres were massive, and as with Elephants, larger mammals tend to have longer lifespans.
Experts reckon that the oldest Paraceratherium could have lived over 80-100 years at least. If an animal lived this long, they would have an extensive knowledge of their environment, remembering the precise location of watering holes for example, even after decades of having not been there since. Indricotheres thrived in Mongolia, Indricotheres such as Paraceratherium, which was the largest of them that we know of.