For a person's 21st birthday you can write a message like, "Congratulations! You're finally legal. Happy 21st birthday!" Another option is, "You’re finally 21 and ready for some fun. Happy 21st birthday!" You should personalize the birthday card by adding the recipient's name.
Consider taking a funny approach by writing a joke like, "Your fake I.D. has expired! You’re 21! Happy birthday!" If the card recipient has a more practical personality, write something like, "A golden era in your life starts now. Happy 21st birthday!" For a person that is religious you could say, "What a blessing to turn 21! Happy 21st birthday!"
Encouragement always works well. Say something like, "You are now one step further into your 20s. It’s a great time for you to figure out what you want out of life. At the same time, though, it’s a wonderful time for you to experience life to the fullest, make mistakes and learn some important, new lessons. Don’t waste this decade. Be young, be wild, be free but be responsible. Be strong! Happy 21st birthday!" Keep the overall tone of the card positive and upbeat. Most people turning 21 years old would enjoy a great laugh from a birthday card while others prefer a no-nonsense approach.