An aging population is only a problem if you are broke and have no capital to use to finance your lifestyle. Japan is nothing like that - they are a rich country with divested interests in many industries that all output world class goods and services. Built by the people now entering retirement.
Worry about Japan when their per-capita starts slipping dramatically.
A still-shrinking working-age population will have to look after a still-expanding retired population.
IIRC (no reference), Japanese pensioners on aggregate have lots of savings. However, they will surely be competing with one another for services rendered by the shrinking labor pool, so one would expect the costs of those services to rise, negating the effect of having lots of savings.
I live in Japan now and, unsurprisingly, the retirement home business appears to be booming, especially in and around Tokyo. But even if there are lots of retirement homes, they will have to be staffed. By robots perhaps?
For one, the people buying those sports cars and big tvs are productive citizens and have decades of work and payments ahead of them...
TBH virtually all developed economies have slowed down over the past 10+ years, I often wonder if this is the real reason, more than just the financial criss. The only spurts of growth seem to be either sudden windfalls from resource extraction booms or big credit runups that tend to leave a big hangover.