The generally accepted rule of thumb is that kids under the age of two need extra protection. A good rule of thumb is to remove the gate once the child’s chin has reached the top of the gate.
In reality, not every child can be described in the same way. Therefore, it is best to remove the baby gate if your child can climb over it or if they have worked out how to open it. The problem is, if your child can rise over the baby gate, or if they have figured out a way to open it, then the best thing to do would be to remove it.