In practical terms you have two options, one is that the trust should come to an end when your youngest child reaches the age of 18, and the other when the youngest child reaches the age of 21. While most parents might feel 21 is too young to potentiality inherit a large sum of money, there are significant tax implications in allowing the trust continue beyond this unless the child will be dependent into the future, and it should only be exercised with caution and proper taxation advice.