A fixed mindset regarding ability or intelligence is particularly unhelpful for children. For example, if they believe that maths ability is fixed, they will quickly give up when they encounter a new maths problem they cannot solve.
On the other hand, if children have the mindset that skills can be managed and improved and are not stagnate, adults, teenagers, and children can understand that they have more significant potential.
We have posts on how to help your child develop a growth mindset; scroll down this page to read them.