Final answer:
Children not full-time students are no longer subject to the 'kiddie' tax at age 19, but if they are full-time students providing less than half of their own support, the Kiddie Tax applies up to age 24.
Step-by-step explanation:
The age at which children who are not full-time students are no longer subject to the "kiddie" tax is 19 years old. However, if they are full-time students and providing less than half of their own support, the Kiddie Tax can apply until they are 24 years old. It's essential to note that there are certain income conditions that could modify the age of applicability of the Kiddie Tax. These rules are designed to prevent parents from shifting large amounts of investment income to their children to take advantage of their lower tax rates.