Very young children (such as those around toddler age) should not have smile makeovers performed. Other than that, though, most people can undergo the process regardless of age.
The more important factor is your oral health; you can only have cosmetic dentistry done if you are not currently suffering from tooth decay or gum disease. Even if you are struggling with these types of dental issues, that doesn’t disqualify you permanently; you may be able to pursue a smile makeover once your mouth is healthy again.
Keep in mind, though, that when it comes to teeth whitening specifically, most dentists recommend waiting until at least age 14.