Because 18 is a milestone birthday, the message inside a card should be meaningful and personal. It can be funny, sentimental or wise, but it should not be cliché or crass.
According to BirthdayMessages.net, birthday wishes for an 18-year-old mark the occasion of reaching adulthood, so sentiments related to age-related privileges are appropriate. For instance, a funny message is, "Happy 18th birthday, when you get to start doing everything legally that you've been doing illegally since you were a kid." Wishes can also be more serious, such as "Though the word 'teen' is attached to your age, you are an adult now. It's time to take on grown-up responsibilities -- you're ready." It is even appropriate to offer advice, such as a general observation that "as you celebrate more birthdays and grow up, make sure you grow up for the better." Sentiments can be more specific, such as exhorting 18-year-olds to remain loving and kind, stay true to themselves and avoid being disappointed by the ugliness in the world.
Birthday wishes can also reference future milestone birthdays, such as an observation that those hitting 30 years old wish to be 18 again. Operatic tenor Robert Breault observed, "In childhood, we yearn to be grown-ups. In old age, we yearn to be kids. It just seems that all would be wonderful if we didn't have to celebrate our birthdays in chronological order," which would be a literary message for an 18th birthday card.