Is isolated extreme absentmendedness in an 18-yr. old suggestive of
ADD in the absence of inattention and hyperactivity?  How much
spaciness can one ascribe to adolescence?

Hello Thurber!

As the mother of an ADHD boy (age 11, going on about 40), I can tell
you that exhibitions of the symptoms of ADHD are anything *but*
isolated.  "Hellooooo....space cadet?" is a phrase uttered to describe
my son several times a day!

You don't say how often your teenager "spaces out", but from your use
of "isolated", I assume it's not all that often.  Though the "extreme"
part probably annoys you endlessly, it sounds like what you have is
just a typical teenager with a lot on his/her mind.

Here's why:

It's easy to understand how ADD can immediately come to mind when
dealing with a teenager.  Consider how one website describes
teenagers:

"Forgetfulness, habit of repeating mistakes, Sharp in dilatory
tactics, dawdling, finding excuses and try to avoid, the assigned
jobs, very slow in grasping, escapism, absentminded etc."

Teenager Negativities
http://www.charminghealth.com/negativities/n-teenagers.htm

...and a mother notes:

"Forgetfulness, irritability, power plays, manipulation, the growing
desire for privacy, the need for peer approval, and the importance of
new friendships were all listed in books that described the typical
adolescent."

Teenagers
http://www.betterendings.org/FosterCare/mfteen.htm

It sure sounds like ADD, doesn't it?  But in both adults and children,
the key word in symptoms of ADD or ADHD is "often", and the symptoms
don't appear "out of nowhere".  Typically, symptoms of ADD or ADHD
manifest in early childhood between the ages of three and seven, and
are not attributable to brain injury or other disturbance.

Lifetips explains the criteria for an ADD or ADHD diagnosis:

"Mental health professionals use a manual called the DSM IV as a
guideline in diagnosing ADD. It lists all of the symptoms of ADD. To
be diagnosed with ADD, a person must have at least six of the symptoms
listed. In addition, the symptoms must have been evident before the
age of 7, be present in at least two settings, and impair the personīs
life. Other conditions (like depression) must also be ruled out."

Guidelines for ADD diagnosis
http://add-adhd.lifetips.com/TipSC.asp__Q__id__E__69737

According to the DSMV-IV, the following behaviors are symptomatic of
ADD:

-- Often fails to give close attention to details or makes careless
mistakes in schoolwork, work, or other activities.
-- Often has difficulty sustaining attention in tasks or play
activities.
-- Often does not seem to listen when spoken to directly.
-- Often does not follow through on instructions and fails to finish
schoolwork, chores, or duties.
-- Often has difficulty organizing tasks and activities.
-- Often avoids, dislikes, or is reluctant to engage in tasks that
require sustained effort.
-- Often loses things necessary for tasks or activities.
-- Often easily distracted by extraneous stimuli.
-- Often forgetful in daily activities.

For a diagnosis of ADD, the child must have had at least 6 of these
symptoms, in at least two different settings, lasting at least 6
months, and they must be "developmentally inappropriate" (interferes
with the child's normal mental development and function).

Sources:

Attention-Deficit Hyperactivity Disorder(ADHD)
http://www.drjoecarver.com/adhd.html

Facts About ADD
http://www.brainevaluation.com/articles/addfacts.html

Research creates new controversy, questions about ADD
Grosse Pointe News, August 17, 1995 pg. 1B by Phyllis Fries 
http://www.nickh.org/add/addfries.html

"Isolated extreme absentmendedness" probably doesn't fill that bill. 
It's likely more the result of being a busy teenager.  At 18, s/he's
doubtless got a headful of school worries, graduation preparations,
college applications, worrying about the future, worrying about
relationships, fretting about social activities, and a whole host of
brand new responsibilities peering back from the end of that long
journey into adulthood.

That's enough to give *anyone* the occasional fit of airheadedness!

Of course, you would know your teenager the best.  If these bouts of
extreme forgetfulness are interfering with his/her ability to
function, it's advisable for you to make an appointment with a
qualified therapist who specializes in children and adolescents. 
*Only* a qualified therapist can make a true diagnosis of ADD or ADHD,
so if there is even the slightest doubt in your mind, get your
teenager in for an evaluation.

I hope this helps ease your mind.  If you need further assistance,
please don't hesitate to ask for clarification.  I'll be happy to
help!

--Missy

Search terms [ ADD ADHD adolescent ] and 11 years as the parent of an
ADHD child.

Missy-ga's answer was excellent.  As someone who also has personal
familiarity with ADD, I would add that one thing that makes diagnosis
often difficult is that many of the symptoms of ADD can also be
indicative of other conditions. For example, someone who is
experiencing high anxiety can show ADD-like symptoms such as
forgetfulness, inability to focus, and extreme procrastination.

I would also emphasize (as Missy-ga mentioned) that ADD by definition
doesn't suddenly appear. Some symptoms are almost always evident in
early childhood (although they may not be labeled as ADD until years,
even decades, later).

My conclusion would be the same as the official answer above: isolated
instances, even if extreme, probably aren't indicative of ADD, since
ADD tends to be life-pervasive. But if the problem is an impairment in
life, a psychologist or other trained professional should be
consulted.

Don't forget that ADD and ADHD can be different. ADHD has H for
Hyperactivity, which is not always present in all cases of Attention
Deficit Disorder. One can have an ADD son who isn't that hyperactive,
but is dreamy all the time. I have the book Driven to Distraction, and
it makes that distinction clear.

But Missy is right in saying other conditions can mimic ADD/ADHD. An
evaluation can make sure what condition your son has.