Its hard to describe such people. Its like schizophrenia. When you read the symptoms, you may easily think that most of the people are schizophrenics. But when you see a real schizophrenic person, you will easily see the difference.
There are some people who are slow minded. They don't need special care to live (they can eat, dress etc by themselves), but they don't think properly, can easily be cheated by someone. They can't be made responsible for important things such as being a commander or being a boss of a company. They usually don't give any importance to earthly things, so when someone say them to give up their property or money, or to lend for a certain time, they don't think that this man may be cheating them and won't give back the money ever.
They are like mentally ill, but in a half degree. There are people who don't give any importance to earthly things with aware, but these are not aware. They are like having a child's mind.
Therefore, we shouldn't give them a full responsibility over property until they begin to think properly. This would be escaping from responsibility, and throwing a child into middle of the wolves. We can foresee that they will be cheated by someone soon, thus lose their property and will become poor and needy. Rather than giving them some money and let them take care of responsibility of that money, we should feed them and clothe them.
Also this verse may include some people which are not old/mature enough to take the responsibility of property such as teenagers or children.
O guardians, do not give to the foolish, the squanderers from among men, women and children, your property, that is, the property that is theirs but held by you. and that's clear. as to your new question, ie,if its ok for us to give unknown stranger who is a beggar !(?), i would say:whatever good you give to the beggars is an act of charity, but be advised, only give them what you think will be enough