Re: Being a host?
I don't think the hosts need to be babies, it's just preferred for the following reasons:
1) A baby can't resist against it, if you're going to try to make a grown-up a host, he can refuse and resist against it. Though this is not the main reason.
2) If you implant a tailed beast into a baby, the baby and the beast create some kind of bond with each other. For instance Gaara couldn't sleep from the start, so het got used to it because he never had slept before. If you would implant it in an adult, he would become insane or something as he slept his entire life and now he can't do it anymore.
3) But the most important reason is the training of both the tailed beast and the host and how they have to work together. As stated in the manga only four people or so were able to completely master their tailed beast. Each beast must have had at least 2-3 hosts by now (which makes something like 18-27 hosts in total) and still only a handful were able to control their beast. With other words most of all the others trained their entire life to control their beast and didn't succeed in it to control it fully.
If you're going to implant a beast into an adult, he wouldn't have much time to learn to control it and then they couldn't be used in battle.
4) Another reason is that it isn't the most wisest thing to do to implant it into an adult. A child you can influence and manipulate and thus making the chance of him betraying his village small. If you're going to give a beast to an adult, the chance that he betrays his village is so much higher as he now got a power that can easily destroy entire armies. A child isn't aware of the strength he wields.
Everyone can become a host, it's just the most beneficial to make babies a host than adults. Less risks, more benefits.