If you are like 15 and you happen to be watching under-age porn, I don't think any serious harm with come to you.
That said, in the UK, the age is 16 and so therefore underage (child) porn is under the age of 16. If you're under this age, if you are watching it, you'd probbably just get a slap on the wrist, however if you were tapping it, you could be done for statutory rape, even if it was consensuel.
In the US, it's 18. So it applies slightly differently.
Either way, if you're a 55 year old man with 25gigs of the stuff, you're gunna get done, if you're a horny teenager wanting to rub one off, I doubt you'd get in trouble.