I saw this again recently and it's always failed to click for me. I can see the young girl but never the old woman..
Then I found the trick: tip you head to the left (right in the northern hemisphere? :-) )and the old woman pops out. Ah ha finally!
This link has several versions.. http://mathworld.wolfram.com/YoungGirl-OldWomanIllusion.html
Normal service resumes..