Most encoders apply a low pass filter to cut off frequencies above 16kHz. Some do it above 18kHz but most choose 16kHz as default setting. This is common in all lossy codecs - mp3, aac etc.
AFAIR, LAME applies at 16 KHz cutoff at 128 kbps, and 17 Khz at 160 kbps. I don't have data about the others, but AFAIK a similar approach should apply.
I'm not able to hear above 16 Khz. I've tried only once an experiment with another person, and they were definitely able to discern the difference.
