I think that assuming readers have finished the game once is good. To be honest, I can't imagine anyone would go combing through a wiki for a game they haven't finished and not expect to be spoiled.
As for born/died vs age/status.... I'm impartial but I've seen wikis who use age/status/birth date or birth years, so we could use all of them. However, birth year may be irrelevant for the most part as we don't have dates and/or years for a lot of characters.
Regarding spoilers, how are they going to be indicated on the forums? As a warning on every page, *spoiler starts here* type warning, "this page contains spoilers," etc.? It'd be important for pages like Shotaro where even their sprite is a spoiler. I may be able to make a ROM themed warning banner.
Additionally I'd like to suggest a "Status" box in the character template boxes to write deceased or alive as opposed to including it in the bio?