Instead of assuming, you should find out first if that's the case, and secondly why.
In my case, it's not a lack of ability but a refactoring of my priorities around my family and young children.
Maybe there are some allowances that can be made. If the person is really the best fit, then you should do whatever is reasonable to get them. You won't even get that far in the discussion if you stereotype based on age.
There are tactful and entirely legal ways to ask those questions without actually asking them:
i.e. "We have a pretty intense timeline right now. Are you able to put in X hours a day for the next X months?"
Honestly, I tend to make some bad assumptions as well. In my past experience, that type of "intensity" isn't because of trying to rush to market or because of pressure from external forces so much as poor planning internally. I have some pretty vivid memories of one company where the reason for the long hours were solely based on bad management decisions around over-catering to customers. Things like "Sure we can have that done in a week" combined with "We don't have time to do proper testing" which resulted in late nights massaging broken data back into the database or performing releases only to realize 2 hours into the process (don't ask why it took 2 hours), that the build was bad.
I've learned to watch for those signs and ask questions up front to suss that information from potential employers.
I think it's generally accepted that, regardless of age, after 16 hours of non-stop work people make stupid mistakes. I like to say "If it has to be done in 10 minutes, can we spend 20 minutes making sure we do it right?" If you can't spare an additional 10 minutes for even a tiny amount of discovery, that's an antipattern.
