You asked

When will my child be old enough for trick-or-treat?

There are children, aged 2-years-old and up, who enjoys dressing up and knocking on every door in the neighbourhood. But your child's understanding of the concept of Halloween actually depends on how well you have made him understand that it is just an act. For some children, seeing someone dressed in scary attire may frighten them, but there are those who would just laugh at it.
Take note that young children may not distinguish what is real and what is not, and seeing their Mummy or someone from the neighbourhood dressed in a scary outfit may not be fun at all. But once your child grasps the concept of why people are wearing scary costumes during Halloween, trick-or-treat will also have a great appeal on them.
You will have to help your child understand what Halloween and trick-or-treat is. You have to make sure that he will be able to enjoy the event instead of being scared of it, and you can do this by familiarising him with your home décors. You can make your decorations a little funny so that he will not notice that it is in fact scary. So when someone knocks on your door, or when he is the one knocking on a neighbour’s door, he will not be scared of what he will see.

More questions

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