Looking at heads of states in non-western countries I'm not sure why you think it's a western thing. African countries got multiple 90+ year olds as head of state for example.
I might've been exaggerating a bit, there's only one right now who's 90+, Paul Biya. Peter Mutharika will be 90+ when he steps down. Beji Esseni was 92 a few years ago. Many are 80+ and will not step down before dying