Yes and no. The drinking of flavored drinks made from pure water and powered juice ingredients are healthy to an extent, but same time they are not. Flavored drinks are packed with all sort of various artificial chemicals that could interfere with proper body functioning, which makes them to be selective of the type of body they are healthy to and the ones they are not healthy to. It is like this, because of the differences in our metabolism, enzymes, hormones etc.
As for the children, flavoured drinks could be very unhealthy for them, as they are ladened with high amount of sugar calories that leads to child obesity. Apart from the sugar content in them, they also have colourants they use in enhancing their brightness. And sadly, these chemicals are all artificial ingredients that are unwholesome to the body. They also have sweeteners in them, which could lead to tooth decay, abnormal bone formation, and many others.
In addition, as for adults, flavoured drink is far considered healthier to them than the children, as they already have their body parts well developed. But that doesn't mean they cannot affect their body functioning too, especially if they are drank excessively.
On a final note, instead of going for flavoured drinks choose to opt for natural fruits, which you can also juice. By doing this, you save yourself all the artificial substances that are in powdered flavored juice.