Technically we know how long Bitcoin will last so at least we can answer that. Of course, elderly might can want to spend their retirement plan to some less riskier traditional investments. But maybe just a small percentage are willing to invest still maybe 10% of their total asset to see how it goes.
@deisik put a good argument, as human, we always seek adventure even as we age. And just imagine milennials and young generations who have accumulated wealth very early, it's possible that they can see bitcoin or crypto in general as a good choice to park their money, IMHO.