Since a 21st birthday is often seen as a transition into adulthood, this speech should feature some sage advice about the adult world that is beckoning. Of course, any birthday speech should also include the speaker's love and admiration for the guest of honor as well.
If you've known the person celebrating the birthday for a long time period, mentioning funny stories about his childhood makes for excellent entertainment, as it's a nice way to look back on the life of someone who is now officially an adult. It's also a good idea to talk about the accomplishments your loved one has achieved in life and what exciting things are in store for him.
Be sure to mention it if the guest of honor has been especially supportive or helpful to you in the past, or you've had lots of fun times with the person. Talk about your loved one's favorable personality traits, such as a sense of humor, a great work ethic or a sense of focus and determination. Express how proud you are of the individual celebrating a birthday, and wish him well as he enters adulthood. Encourage the guest of honor to continue seeing the world in a positive, childlike way for a touching ending to the tribute.