It isn't about if you react or not so much as how you react. Firstly in the reporting of these figures the Daily Mail vaguely mention that "Charities said easy access to online porn meant thousands of schoolchildren barely into their teenage years are getting their sex education online."
It is up to the young person to decide if they are ready to become sexually active or not - with all the right education. Such education would also - hopefully - impact on the high percentage of those under age being treated for sexually transmitted infections who were abuse victims rather than very young people choosing to have sex. Knowledge of who to report abuse to and vocabulary to help express yourself when dealing with such an act beyond your maturity level (and who is mature enough to deal with sexual abuse?) should enable those in need to seek help.
Doctors who treat under age children for STIs say that realistically the overall figure of just over 5000 is probably far below the actual number. Due to shame and fear about seeing who is probably the family GP about any intimate problem many children won't go and get checked out. Rather like adults. Interesting, no?
There's not a chainsaw maniac on the loose and there's not maintenance guy looking to violently spaff in every orifice he can.
The answer is because we don't talk about contraception and therefore the impression is that it is not necessarily. Coupled with the human tendency to believe things such as STIs, accidental pregnancies and abortion don't happen to them means we have more and more people taking more and more risks and we need to figure out not if porn is to blame but what is lacking - education or esteem.
Every time there is a panic regarding young people and sex we jump up to blame how raunchy society is. I don't think there's evidence of society de-raunching just being differently raunchy. Which means we have to create a society that can talk about its raunchiness rather than pretend it isn't raunchy. And if we can't do that then we need to insist that every porn film ends with the words "based on a true story" and "use a condom."