I chose 10-20 years because, though every action (or non-action) has an effect of some kind, I don't think that anything I did younger than 10 or so would have effects that would make the ATL of my non-birth different from OTL.
Now, in middle school through college, I imagine I've had many opportunities to affect people (relationships with others, that editorial I wrote for UGA's "Red and Black" newspaper, this board, my other writings, etc). Those slightly bigger things could butterfly down the road (my editorial might have convinced enough people to not support John Kerry to seriously affect his chances of election--in the ATL, some ex-Kerry supporters would stay Kerry supporters, campaign for him, etc).
Of course, I will concede that my having such a great influence by writing an op-ed in a college newspaper is likely a pride fantasy, but one never knows. Unknown people have influenced known people throughout history...abusive boyars affected Ivan the Terrible, someone much like "Max" in the titular film could have turned Hitler from anti-Semitism, etc.