I have basically raised and helped to raise a few foals and youngsters. The ones I have raised almost completely by myself are now yearlings, and for now they are a pretty decent bunch in terms of manners. I'm probably more than capable in a way, but the idea of having my very own youngster makes me break out in a cold sweat. There are lots of phases between a foal being born until the age when they are ready to be broken. Sure there are some that are a gift from heaven and the stereotype of an easy horse in the making, but nevertheless they will act like horses and try their place in hierarchy - some more than others. Personally, I have got nips, strikes, and climbing on my back (colt) or charging at me to play when I turn, and all kind of other more subtle signs of domination. And you will have to know how to correct them immediately. Every time you are with the foal, you are training him to become a good horse citizen. Everything you do will affect what she will become.
I do confess that I personally just prefer older horses because I like the stability. With youngsters there are hormones, temper tantrums through teenagehood, and a lot of things questioned - and a lot of repeating. I wouldn't advice a first horse to be a weanling, unless the person has some experience being involved in raising foals and youngsters.