I'm a geezer (50) at Facebook. It's not the hours, as Indiana Jones might have said had he been a programmer, it's the lines. (I don't know what that means--it's just too good a line to pass up.)
There are ways I provide leadership and ways I accept leadership from my peers, and the mix is different than it might have been twenty years ago. My code reviews focus on strategic issues ("could this be divided up into N diffs?") instead of sorting out complex behavior. I pass off many of my ideas since I'm slower than I was at implementing.
I am as engaged by this job as anything I've done for fifteen years. A positive, curious attitude goes a long way, as does the expectation that I need to earn the respect of my team and give them respect where due in return.