I see quite a few 50 year olds in larger, more established companies. Older people don't join startups because they can't afford to, as a general rule, but the rest of the age skew that exists is as much because the current round of technological skill set is rooted in the GUI and Internet advances of the 90s as much as anything. That ends up putting a soft bound on career length based on if you did your primary learning before or after that.

Anyone older cut their teeth on assembly or C, NetBIOS, and CLI and may or may not have stayed current afterwards. Consider that the industry was a lot "older" (just look at old pictures) until those technological shifts happened and shook out all the people that learned in the 60s or 70s.

If something like quantum computing or a parallel functional computing revolution or VR/augmented interfaces or anything else grossly disruptive to current development practices comes around, we're all going to be in the same boat: learn or die. Getting through that is not so much a factor of age as the circumstances around age and one's willingness to keep at it.