As with many things in poker, "it depends."
It depends on many factors such as situation, effective stacks, tournament stage and so on, but generally A9o should probably be a fold. The lower Ax hands (certainly A9o and below) play similarly.
Many players get excited if an Ace hits when they hold A9o, but even here I'm not thrilled and this is why I seldom play A9o for its objective value. Put bluntly, even if hit, we are dominated by not only the premium AK and AQ hands, but even crushed by AJ and AT similarly.
The danger of playing the really low Ace-rag hands is that it comes down to your lousy kicker and you lot the hand or that your lower card may get counterfeited if paired (three pairs with your the lowest).
I hope you don't get the idea that I'm cynical to A9o, but we must be realistic about the danger of this hand not being best if we do hit an Ace and more often then not, we will not connect with the Ace and be in even more trouble.
Actually I love A9o! My first ever freeroll event was the cardschat 30 Day course freeroll and if I recall correctly, my final holding was Ah9c (A9o!). Naturally, I like this hand enough to smile every time I see it come up, but that doesn't mean I don't fold it as well