Can you develop add as an adult but never had it as a child?

No: Add is a disorder that begins in childhood, but may not be recognized into adulthood. Careful investigation will often reveal childhood symptoms that were missed. More intelligent individual may not manifest difficulties until later in life. The exception to this is head trauma. Damage to the premotor cortex can produce symptoms that mimic add.

ADD is genetic: As the md mentioned , it is diagnosed before seven, but it is the most fascinating brain disorder, there is attention deficit only when they are bored, they are attention surplus when they are excited.As adults the part of the brain which controls the movements is called substantia nigra matures , so they r not hyper, just fidgety, but still forgetful, disorganized, .Read russel barkley, for adult add.