Unfortunately, mental health disorders like anxiety don't have a minimum age requirement. These disorders can be difficult and traumatizing both for the children who experience them and for their parents. Additionally, dealing with disorders like anxiety in children is even more tricky than usual because, often times, it may be hard to figure out if your child is suffering from anxiety or from another condition. In children, the symptoms of mental health disorders like anxiety often overlap with symptoms of other disorders, and it can be very easy to mistake anxiety for a learning disability or an attention deficit disorder.
For example, your child may think that they are not good at math: each day when it's time for a math lesson, your child may complain of a stomachache in anticipation. They may ask to go to the school nurse, and then miss the lesson and struggle to complete the worksheets that are sent home for homework. Anxiety can look like your child is struggling with a particular subject, but it is the thought that they can't do math that leads to losing out on instruction, rather than a true learning disability.
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https://www.anxiety.org/causes-and-symptoms-of-anxiety-in-children