More seriously I think something that can happen with older folks is they might fall into working in a single place with a single set of tools for a very long time so when they transition to something different it may be hard to adjust. Not necessarily because “they are old” but because if they did one thing and only that thing for 15 years it’s hard to change.
They could have had those roles already. Once you're in those advanced IC type of roles, there aren't really a lot of places that you can go. A lot of the time that role depends on your domain experience within the company -- Usually to "move up", you have to go down a role to a bigger/more-challenging company.
Thus at BigCo, hiring senior devs of advanced age should make sense to do.
This is entirely dependent on the company. Netflix has one level "Senior Software Developer". My former employer only went up to Principal Developer. My current gig has one Senior Staff Engineer, me. The thing is, I didn't get better in the month between leaving my last gig, and starting this one. So these titles are a bit ambiguous.
Many companies don't have positions titled staff, principle, etc. A lot just stop at senior, or use numbers, etc. My current shop has SE1,2,3,4,5,6. 4 is what most tech companies would call a staff engineer, 5 a principle, and I'm not sure if we actually have any 6s at the moment.
More seriously I think something that can happen with older folks is they might fall into working in a single place with a single set of tools for a very long time so when they transition to something different it may be hard to adjust. Not necessarily because “they are old” but because if they did one thing and only that thing for 15 years it’s hard to change.