I was completely ambidextrous until I was about 5 or so, when I was told in kindergarten that I should pick a dominant side and stick with it (went with my right hand since that was the majority).
I notice I still tend to do a lot of things with my left hand like eating, using certain tools/scissors, and even ocassionally using my left hand on my computer mouse. Writing, though, I can't do anymore, which is a little disappointing.