Whoa, didn't expect the announcement to come so soon. Of course, the sound bytes were everywhere, but even then, this was a surprise announcement.
So, the Tim Cook era lasted 15 years (2011 - 2026). He's 65yo, and he could have easily hung in there for a few more years. But I believe he's leaving at the peak -- both Apple's and his own -- and this might be the best time to leave, rather than being forced out (as many too-long-in-the-tooth CEOs have been) when the company inevitably grows slower, or has a crisis.
Ternus is 50-51 yo, roughly the age when Cook himself took over Apple. There the similarities disappear. Ternus is a HW guy through-and-through. I hope he has solid SW and Design team with him. He's gonna need it, given all the big/small design snafus in the recent past. [Not including Mac Neo in there, which looks stellar by any means]
Wishing him luck; he's gonna need it. (and me too, my $$$ are invested in AAPL, and I ain't selling anytime soon, so well ¯\_(ツ)_/¯ )
Granted, 65 is probably a milestone for anyone. I think BMW has a hard stop for its execs at that age