Mineral constantly comes in and out of the enamel crystal matrix due to acid challenge (which occurs for a variety of reasons), and including fluoride when it goes back in creates a more acid resistant crystal.

This occurs no matter the age of the individual. Systemic uptake is something to be mindful of at young ages, so it’s actually important to have not too much when younger, but you can go up to an adult dose past the age of 7.