This is an interesting question, because in our minds there is no doubt that it is possible. From our experiences, we have not had any instances where children have not begun to demonstrate thinking skills once we start working with them.
In fact, even the most reluctant of kids mid-way through one of our workshops opened up with, "I have an idea" and then refused to stop contributing this thoughts! We work with them in a "play" environment and reflect with them freely, this leads to a learning space where the facilitator ends up learning as much from the children about the activity as the other way round. This builds a lot of confidence in the child about his ideas and his abilities. When the learning space is always about open ended solutions, there can be many winners and confident thinkers are born!
The question regarding age is a very interesting question and a topic for another blog...keep watching this space for more thoughts from Sparkling Mindz team!