So you do believe that terrorists are born, not made?It cannot be prevented.
People are alienated first before they resort to public complaints, demonstrations or further, more violent acts against the 'system'.People will and do go to great lengths to do what they believe for a cause.
Please explain then, as you think you are an expert, how the country will stop a 15 year old, who is off the radar of any terrorist watch list, can be prevented.
This will be interesting.....
Prevent the alienation and you prevent the subsequent consequences.
Failure to prevent alienation means that you need to employ other measures, prevent further radicalisation, utilise efficient and effective passport control systems, develop better international cooperation in recognising and returning potential terrorists.
There are a multiplicity of measures that could be taken. Even whether 15 year olds should be allowed to travel without parental consent. (consent and with accompaniment are different)