When to use words instead of numbers?

When do you use words instead of numbers? For instance, when writing ';My son is 12 years old.'; Should I use the number 12 or write out twelve? And when do you switch from numbers to words when writing numbers? I hope that made sense.When to use words instead of numbers?
i think if it's more than ten you can write the digits


not sure thoughWhen to use words instead of numbers?
You always spell in formal writing/papers/letters/ etc.