When the Chief dies, we have to find a new one. With 90 villagers, this is tedious. I'm wondering if there are any "rules". Like, will the new Chief have to be someone who wasn't around when the old Chief was alive?

I have not found that to be the case. Right after my first Tribal Chief died, his successor was five years younger and had been around the whole time. I think you have to pretty much go through villager by villager until you hit the jackpot.

Yes, that's what I think too. No shortcut. It was easy to find a new Chief when the tribe only had 9 members - but with 90...

And I do want a Chief around to educate the kiddies.

You can try pausing the game and then pile all your villagers on the robe and then unpause the game.

is there a minimum age for the tribal chief?

2, I guess. If you try all tribe members, but there is a baby, that baby might be chief when he/she turns 2.