Children as young as 11 are obtaining gun certificates from Sussex Police.
And youngsters of just 15 are being granted other types of firearms licences.
Lewes MP Norman Baker highlighted the worrying facts to lobby for more gun control.
He said he would not criticise Sussex Police for issuing the licences because they were merely following rules.
But, he said: "I believe it is inappropriate and irresponsible to allow a child as young and impressionable as 11 to legally possess a lethal weapon.
"Whilst I believe older children should not be completely denied access to firearms, for instance in a safe and responsible environment such as a rifle range, I can think of no convincing reason why young children should be licensed to use lethal weapons.
"I find it all the more disturbing where such children are then encouraged to participate in bloodsports and are, consequently, being desensitised to the suffering of animals rather than being taught to respect wildlife."
He is lobbying for the introduction of a minimum age limit of 14 for using a gun at shoots.
Sussex Police confirmed Mr Baker's findings.
A spokesman said: "This is a shooting and fishing county which has a low-level of gun crime. With the backing of the Association of Chief Police Officers, we believe if you train people young in how to properly and responsibly handle firearms then they are in good stead for the rest of their lives."
He said if youngsters were seen with any firearm unsupervised or acting in any irresponsible way, police would act.
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