I answered as close as I can remember, but I think touch typing is more of something I kind of picked up as time went by, rather than something I specifically learned at one point in time. I remember pushing myself to practice touch typing at various points, but the general recollection I have is that I didn't really practice in a focused, systematic way, and yet now I can type this without needing to look at my keyboard (and in fact, when I look at my keyboard I'll be likely to make more mistakes).

So I probably picked it up in my early twenties with a lot of typing of homework and essays and posts on forums.