Given that Dante (who was no theological slouch) put adults there, sure, there's a possibility for virtuous heathens to attain Limbo. The Church has never definitively ruled against the possibility, but, as josefa mentions, it would truly be a feat for them not to commit at least one mortal sin in their life.
My cousin was mentally disabled and died outside of the Church at 30, and I'm certain he's in Limbo or, God-willing, in Heaven (if my relatives had him baptized, which I don't know for sure). So it's a definite possibility for mentally disabled adults to be placed in Limbo.