Shanahan, IME, it is better for them to go through it younger, as homesickness at 7 or 8 is generally easier to fix - hug from friends, cake, and distract new fun thing to do and it's all gone. 12-15 yr olds tend to wallow and are also more likely to be embarassed about it. And fellow 7 yr olds will not be surprised about it, and will be sympathetic and try and help, 12 yr old peers may well snigger (not all of them, but in a group, there will always be some sniggerers). Homesick teenagers in particular are a nightmare, they get each other down, I had 5 x 15 yr olds all sobbing in their dorm in Switzerland, cos they all missed their boyfriends. Much easier with Brownies - 'feeling a bit sad, let's go and help Snowy Owl make tea', and it's all forgotten.
This is just from my experience running Brownie/Guide events, younger ones get homesick and it lasts 3 minutes then is forgotton about, older ones it lasts all day and beyond, and tends to be something they remember.
Obviously it's each person's individual choice, but just thought I'd respond to your point about it being better at 12 than 7.
D