My favorite one is pictured here. It's a 61 year old grandmother and her 12 year old granddaughter. It's so effective because they share so many facial similarities, i.e. face shape, eyes, nose, etc... Because of their similarities, it looks as if it's the same woman with half her face aged or rejuvenated by almost 50 years.
I encourage you all to take a look at the rest of his work. Surely, you will find something else that you like. He also makes side-by-side comparisons between sisters, brothers, twin siblings, parents and their offspring whether they be male or female. It's a pretty cool project. You can see the rest at http://genetic.ulriccollette.com/.
You should also take a look at this video posted on Vimeo that features my other favorite pairings: