The Olsens Refuse to Grow Up
"Neoteny" also called juvenilization, is the retention, by adults of traits previously seen only in juveniles. In neoteny, the physiological development of an animal is slowed or delayed. Ultimately this process results in the retention of juvenile physical characteristics well into maturity. -Via Wikipedia

The Olsens also display the characteristic "Mickey Mouse" heads associated with neoteny.

The Olsens Wear Tiny Dead Animals
Mary Kate Olsen Is Stoned
Olsen Nose Job Charting
The Trollsens