If there are to be changes to child benefit — and given the financial situation there need to be — then removing it from households with a higher rate taxpayer is a better move than stopping it at 16.
Child benefit ending at 16 would send out a message that at 16 a child should start earning its way in the world. This would, for obvious reasons, have a negative impact on social mobility as it would likely lead to fewer children staying on at school post 16.