How to find the inflection point in a noisy curve?
I think Jazzmaniac's suggestion concerning Savitkzy-Golay filters makes a lot of sense. However, I do not suggest to simply smooth your data using these filters. What many people are not aware of is the fact that there are differentiating Savitzky-Golay filters, which smooth and compute the derivative at the same time. Such filters are implemented in Matlab: check this

You can use the smoothed derivative to detect your desired starting point. It will still require some heuristics and tweaking, but I do not see any other more straightforward way to do it.