Yes, but lets take a step back. A 15yo educated person in the western world is capable of telling right from wrong, good from evil. They are old enough to take resposiibility for their actions, they are above the age of criminal responsibility.
They leave the UK, their family and head to a country to join a terrorist organisation for whatever reason. They've disenfranchised themselves from the country they were born into, their family etc of their own voliation, and if not haven't alerted authorities to they've been co-erced into going. Young people get lambasted for supporting the EDL, etc, but ISIS are as bad, if not worse. What's the difference.
Being 15 is great, getting boozed up with your mates, smoking, joy riding, all part of misdemeanour of youths. Joining terrorist organisations....not so.