Because I want them to have more fun before becoming the young adult and for the obvious reason, changing their appearance to be more teenage looks (height for example). Instead of developing the babies interaction, they could possibly change the YA and adult interaction towards the babies... Emm.. Not sure if it is suppose to be the other way round.. But, I would prefer Teen to be revamped. It is much better if they are gradually changing in term of appearance (although, it sounds impossible)..