Poppy's lore claims that she was around when Demacia was "an ambitious new nation". Seeing as Fiora's lore is full of stuff about laws and traditions (which would require that Demacia be at least a century old), this means that Yordles must have longer lifespans than humans.
But then again, Riot absolutely hates talking about Champion ages because "CONTROVERSY!".
Incidentally, if Poppy really is hundreds of years old, she must be pretty dang dense not to realize by now that she's the hammer's intended owner. Guess she fills the "[Idiot Hero](http://tvtropes.org/pmwiki/pmwiki.php/Main/IdiotHero)" trope appropriately.