More the late 70s for me. I was always tall, and so I was asked all the time for my age from around 13 yrs onwards, to be granted a kid’s under 16 discount. We don’t get carded as such here in the U.K., but being forced to take a birth certificate with me every time I went to watch a film became tedious. That, coupled with the ease of getting in, meant that instead I watched X or 18 certificate films from age 14, such as Alien, Apocalypse Now, Death Race 2000, Damien Omen 2, The Hills Have Eyes, The Warriors, Quadrophenia, and never got carded once to check if I was underage. Then again, everyone at school did exactly the same: our cinemas were very lax.
The age restrictions don’t appear to be that severe now, for example, Deadpool 2 and Suicide Squad are only 15 certificate here, and far more extreme than many of the X / age 18 films I saw back then.