The age at which concrete should be before you can lay a smooth compound on top of it can vary depending on several factors, including the type of concrete, curing conditions, and the specific smoothing compound you plan to use.

In general, concrete should be allowed to cure for a minimum of 28 days before applying any type of topping or overlay, including smooth compounds. This is because concrete continues to gain strength and undergo chemical changes during this curing period.

However, there are situations where you may be able to apply a smoothing compound earlier, such as after 7 to 14 days, if the following conditions are met:

1. Concrete Strength: The concrete must reach a certain level of strength before applying the smoothing compound. A common guideline is to have a minimum compressive strength of 3,500 psi (pounds per square inch) or as recommended by the manufacturer of the smoothing compound.
2. Moisture Content: The concrete should be sufficiently dry. Excess moisture can interfere with the bonding of the smoothing compound. Use a moisture meter to ensure the concrete’s moisture content is within the acceptable range.
3. Surface Preparation: The concrete surface should be clean, sound, and free of contaminants, dust, dirt, and any previous coatings or sealers.
4. Compatibility: Ensure that the smoothing compound you plan to use is compatible with the type of concrete you have, and follow the manufacturer’s recommendations for application.

It’s essential to check the specific manufacturer’s guidelines for the smoothing compound you intend to use, as different products may have slightly different requirements. In some cases, they may offer products specifically designed for use on new or green concrete.

Additionally, local building codes and regulations may also dictate specific requirements for topping applications, so be sure to consult with local authorities if needed. Always follow the manufacturer’s instructions and best practices for the particular smoothing compound you choose to ensure a successful and durable finish.