Well if you are in a dojo where they don't wear gi pants then you better start at day one. Other wise start as early as you can because the damn things are hard to learn to move in with out breaking a toe or pulling a hamstring.
Personally I hate gi pants especially when I am doing iaijutsu. Also in Aikido when doing shikko. I find biking shorts to be the best so far. Nothing under it would be great, but I would get taken away.