The first '18' movie I saw at the cinema was Total Recall, I was still a little underage, 16 or 17.
On video rental, my first memory of an '18' is seeing First Blood at the next door neighbour's house(we did not have a video), I always remember me and my wee pal(who was 2yrs younger than me, he would have been about 5yrs old at the time) were laughing about the fact that the local priest walked in on us watching FB, he was round to give his old great-grandma holy communion and maybe an excorcism(she would not shut up sometimes).