I would agree with pjz99, the shorter one does look a bit older to me.  Would also agree about the hips being just a tad more rounded.

My husband and I understand totally. He has been asked twice recently if I am his daughter. And we are only 1 year apart.  He gets dirty looks in public as well and occasionaly told he should be ashamed of himself for messing around with that "girl",lol.
When I was 20 and we were just married, it just so happened that he and our roomate were deployed (US Navy) at the same time. About 2 weeks later, someone knocked on my door.
It was Child Protective Services asking my where my mother was and why was I home alone,ROFL! Someone had actually filed a report with them. I can laugh about it now, but I sure wasn't happy about it then. 

Age is very subjective, I don't envy the mods jobs of trying to calculate the age of 3d meshes.