Tough one. Wado is (or should be ) less demanding on the body than eg
Shotokan or Tae Kwon Do, however the level of physical intensity varies
significantly from club to club, and the sensitivity of the instructor.
Wado should be about relaxed concentration, but getting to the stage of
proficiency requires a long apprenticeship in basic training, the
syllabus of which seems more fitted to the younger generation. Many of
the required movements and techniques (eg mawashigeri and sokutogeri)
can be very demanding on some people and, in most organisations, you
have to show a level of proficiency in such as these before you can move
on to the more subtle, and relaxed, techniques. Sadly, this 'level of
proficiency' for many is epitomized by lots of sweat, tension and
energy. This may be too much for your back to cope with, but the only
way to find out is to maybe give it a try.

Everyone's body is different, and not everyone is naturally gifted to be
able to perform side and roundhouse kicks to head height, despite the
insistence of the instructor. Excessive muscular tension in other
techniques can also be damaging to joints, especially as the muscle
themselves become more powerful from the training.

Most people who have trained a long time will have had plenty of
injuries, and have their own tales to tell. My own is that beyond 40 I
wasn't able to train the way I did for the previous two decades, and
along with several surgical interventions for discs and facet joints,
and vast amounts of physio/osteopathy/chiropractic I've been able to
continue for another 15 years BUT I can't train like a kid anymore, and
the stuff I do has to be far more like the Chinese 'soft' styles.

It's worth giving it a try, but the trick will be to find out early
where is the dividing line between enough and too much, and not push
yourself too hard. By the way, if you want to protect your back through
your training (and later in life) I'd strongly recommend Pilates, or at
least Core Stability exercises - you'll save yourself a lot of grief,
and it will benefit your karate progress significantly.

Good luck

Neil Ellison
Teesside, UK