A lot of these questions are answered in the Lit, which obviously is not canon so you're free to accept it as you please, or not.
Any ideas how the Symbionts reproduce, or how long a symbiont can live for?
The novel Unjoined
claims that the symbionts (or Swimmers as they call themselevs, as opposed to the Walkers) can grow to many thousands of years of age and as massive in size as an Earth manatee.
Dax, as an eight-lifetime 300-year-old symbiont, is considered young to these older symbionts. They can continue to join to the humanoids for several more hundred years than this (for example, Odan was already on its eighth host when Dax was only on its first, explaining why the Odan symbiont looked different from Dax - it was simply older), but eventually there must come a point when the symbiont is physically too large for the host to accommodate anymore.
It is at this point that the symbiont returns to the pools and where, given the chance, it will grow into one of the ultra-old ones. These old ones also lay eggs (I think) which some middle-aged symbionts who act as caretakers for the old ones then carry up to the surface, which is how we get new symbionts.