Well, my parents did, for example. I was pretty "rational" at an early age and my parents misinterpreted that as maturity, which at some points got me into situations I shouldn't have been expected to be able to handle at that age.

It's not unheard of for parents to want their children to consider them peers rather than parents. The problem is that you can't really be both. The roles and responsibilities of a parent and a peer in a child's life are very different.

Also, if you signal to your child that you would prefer them to act more maturely, that child can become pretty good at faking maturity without the emotional and psychological stability that would come with it if it came naturally. As a parent you have to look past that superficial behaviour.

And for a less personal example, just look at child prodigies, child actors, child pop stars, or for a particularly disturbing example, child beauty pageants. That a child shows a particular trait normally associated with maturity or adulthood doesn't mean the child should be treated as an adult. Yet, this is exactly what happens to many such children who happily play along because their make-believe is positively reinforced by the adults around them.

There are plenty of people willing to believe an eight year old is "very mature for their age" when they should know better. Heck, the entire premise of NAMBLA is a reflection of this (simply taking the illusion to its disgusting logical extreme).

Sometimes children can mature faster in someways than others, and parents can 'forget' that their child is still only X years old.