Wishing ​Birthday 18 needs to be special as they are entering adulthood. Here are some wishes:

On this special day, I hope God blesses you with everything best.
Being an adult definitely suits you. Either kid, teenage or adult, you will be the same amazing person to me.
You are truly an amazing, kind and fun person. I wish your future to be as bright as you.
May you get an abundance of happiness. Enjoy your day to the fullest and have the best celebration ever.