2 Added some sources
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I can't remember the exact details, and I don't have a source (I'll find one when I get home from workUPDATE: Found a source!), but I recall reading once that as

As humans age, our range of hearing changes slightly and it gets harder for us to hear high-frequency sounds. As a result, there are certain tones (between about 16 and 20kHz) that can only be heard by teenagers - these have been used to create special sonic alarms"mosquito alarms" to deter teenage criminals and loiterers, as well as "Teen Buzz" ringtones that can't be heard by teachers.

This change in hearing range - known as presbycusis - is the only way I can immediately think of that you can reliably distinguish between children and adults, in order to kill the latter while sparing the former. Either:

  • A tone that only adults can hear, and that kills them, or;
  • A tone that only children can hear, and that somehow spares them from whatever kills all the adults

Since you've stated you want the surviving children to be able to grow up, the tone should only play once, audible across the planet, for as long as it takes for all adults to die. After that, it never plays again.

I'm sure this can be fleshed out more but I believe it's a good starting point.

I can't remember the exact details, and I don't have a source (I'll find one when I get home from work), but I recall reading once that as humans age, our range of hearing changes slightly. As a result, there are certain tones that can only be heard by teenagers - these have been used to create special sonic alarms to deter teenage criminals.

This change in hearing range is the only way I can immediately think of that you can reliably distinguish between children and adults, in order to kill the latter while sparing the former. Either:

  • A tone that only adults can hear, and that kills them, or;
  • A tone that only children can hear, and that somehow spares them from whatever kills all the adults

Since you've stated you want the surviving children to be able to grow up, the tone should only play once, audible across the planet, for as long as it takes for all adults to die. After that, it never plays again.

I'm sure this can be fleshed out more but I believe it's a good starting point.

(UPDATE: Found a source!)

As humans age, our range of hearing changes slightly and it gets harder for us to hear high-frequency sounds. As a result, there are certain tones (between about 16 and 20kHz) that can only be heard by teenagers - these have been used to create special "mosquito alarms" to deter teenage criminals and loiterers, as well as "Teen Buzz" ringtones that can't be heard by teachers.

This change in hearing range - known as presbycusis - is the only way I can immediately think of that you can reliably distinguish between children and adults, in order to kill the latter while sparing the former. Either:

  • A tone that only adults can hear, and that kills them, or;
  • A tone that only children can hear, and that somehow spares them from whatever kills all the adults

Since you've stated you want the surviving children to be able to grow up, the tone should only play once, audible across the planet, for as long as it takes for all adults to die. After that, it never plays again.

1
source | link

I can't remember the exact details, and I don't have a source (I'll find one when I get home from work), but I recall reading once that as humans age, our range of hearing changes slightly. As a result, there are certain tones that can only be heard by teenagers - these have been used to create special sonic alarms to deter teenage criminals.

This change in hearing range is the only way I can immediately think of that you can reliably distinguish between children and adults, in order to kill the latter while sparing the former. Either:

  • A tone that only adults can hear, and that kills them, or;
  • A tone that only children can hear, and that somehow spares them from whatever kills all the adults

Since you've stated you want the surviving children to be able to grow up, the tone should only play once, audible across the planet, for as long as it takes for all adults to die. After that, it never plays again.

I'm sure this can be fleshed out more but I believe it's a good starting point.