I did not exactly start collecting watches that early (in lack of funds), but the first clock I took apart in the age of 9 years was the Junghans Trivox alarm clock of my parents. They were not really happy about this, packed the parts together and locked me away in a small cabinet without windows together with the pile of parts. And announced that they would only let me out again after I had reassembled the clock so that it works perfectly. Actually I suspect that they expected me to start crying after an hour or so and begging to let me out again without having fulfilled the task. And that this would teach to never again take things apart.
But. I spent about three hours reassembling the clock in silence and without any distraction. One of the finest memories of my childhood. It did, of course, work perfectly. I was let out with no words. And continued talking things apart for investigating how they work
Here is a nice video of the movement with alarm in action. There are three stages, first a toc toc toc, then a ding ding ding, finally the riiiiiiiiiing. In the video the first two stages are not distinguished in absence of the bell. One might understand that it took me about 3 hours to reassemble the totally dissassembed clock.