Translation:The association was founded only five years ago.
Well, Esperanto passive participles are more about aspect than tense.
So you really only have a simple past "estis"; the -it- in "fondita" only says that the founding was finished ("founded" as opposed to "still being founded" or "about to be founded").
You could translate it as either "was founded" or "had been founded" because "estis" can be either "was" or "had been".
I guess it depends on how you parse it: do you see "antaŭ nur kvin jaroj" as an attribute of "fondita" or of "estis". How would you translate "La asocio estas fondita antaŭ nur kvin jaroj" (with "estas" instead of "estis")? If I say "La asocio fondigxis ..." it presumably also implies completion of the action, so there is some link between past tense and aspect when not explicitly given, or maybe that is my own interpretation. Confused now. :-/
How would you translate "La asocio estas fondita antaŭ nur kvin jaroj" (with "estas" instead of "estis")?
That would make little sense to me. "The association is founded only five years ago". You have "is founded" (present tense) and "antaŭ kvin jaroj" (time in the past).
You could say "La asocio estas fondita sur la principoj de ..." (The association is founded on the principles of ..."), perhaps.
If I say "La asocio fondigxis ..." it presumably also implies completion of the action
I could read it as either - continuous ("the association was being founded") or completed ("the association was founded / had been founded").
The sentence "La asocio estas fondita antaŭ nur kvin jaroj" could perhaps be parsed as copula ("estas") and adjectival phrase ("fondita antaŭ nur kvin jaroj"); compare the construction "La asocio, fondita antaŭ nur kvin jaroj, nun havas mil membrojn", where the same phrase ""fondita antaŭ nur kvin jaroj" functions like a single adjective in describing a state in the present, not an action in the past. It feels a bit slippery, like how in English "The fish is cooked" can mean either that the fish is in a state of having been cooked, or that (in some non-specific time-frame) there is cooking going on of that fish); the "is" can be seen as a copula or as part of a compound passive verb form.
Thanks for your reactions by the way, I am enjoying the process of trying to wrap my mind around the logic behind the semantics and constructs!