Children move through broadly predictable developmental stages, and behaviour that looks baffling is often a normal sign of growth. Toddlers do not yet understand intention: they grab, bite and pull things apart out of curiosity, not to cause harm. By around nine, friends start to matter more than parents, a shift that continues through adolescence as young people seek independence.
The ages are only a guide. The order in which children progress through the stages matters more than the exact age at which each one happens.
via An Age by Age Guide for What to Expect From Kids & Teens | Hey Sigmund