>Venice was run by very old men. It was common for the Doge to be in their 80s. Meanwhile, many of their neighbors had kings who were very young, sometimes teenage boys.

But those kings were (legally) absolute monarchs, while Venice was (somewhat) a republic. This isn't a trivial distinction. The young kings of the various Carolignian successors also tended to inherit their titles when their fathers were killed, while Venice occupied a highly defensible geographic location (a swamp) which supported institutional continuity.