There really is no age requirement on clipping. Well, let me amend that. A bird should never be clipped until it has fully fledged, which is to say that he/she has fully mastered flight, as doing otherwise will likely hinder psychological and emotional development. After fledging, however, people either clip or don't... depending on their preference.
Personally, I never clip. But this carries with it the responsibility of recall training my birds, training them well in flight, making sure all toilet lids are down, having the stove top off when they're out, making sure they are well acquainted with every wall, door, window and mirror in the house, and training everyone in the household to be sure the birds are in their cages before opening a door.
It seems like a lot, but once you get used to it it's like a part of everyday life. It's 2nd nature now for me to look over at the cages before reaching for the door. Anything else would feel weird at this point.
And safety-wise, as April and Desi both mentioned, you should never risk taking even a clipped bird outside without a harness. Some on this forum practice free flight with their birds, but such is always a risk. And a bird should be very thoroughly recall AND startle trained before attempting that. (Too many raptors around here for me to ever consider it with my ekkies!)