It depends on the two people involved. Most 24 yr olds I know wouldn't date someone that young. At 24 you have been thru college and/or have been out on your own working and have matured quite a bit. You have a good idea of who you are. At 18, you are still more of someone's child and less your own individual. You are just beginning to find out who the real "you" is. You still have your mistakes to make and your lessons to learn.