1. Would there by any procedures to prevent 15 year-olds from accessing pornographic holo-fantasies like Vulcan Love Slave Part 2?
Only if we assume that the enlightened humans of the 24c have the same narrow-minded and over-protective of sex values parents in the Western World have today -primarily Americans.
I suspect they don't.
It's more likely they way their children are raised -in an open minded enviroment that isn't overly protective- their children would have no interest in such programs as their parents haven't created a mistique about sex.
Teenagers today are interested in porn in its various form simply because their parents hide such things from their kids. What happens? Kids being kids rebel against their parents and seek out porn to see what all the fuss is about.
No mystique, no drive to seek it out.
And even IF a child in the 24c wanted to seek out and use holographic porn we can probably also assume that the 24c parents would be Ok with it in an, "Well, it disapoints me, but do what you want." kind of way. Also recall that Worf allowed Alexander to attend something dangerously close to a strip-tease in that absurd Lwanana episode, granted that was a Klingon child rearing situation, but it may be an example of how such matters are looked at int he 24c.
2. The doors to that thing never seem to be locked. Do you suppose it ever happens that anyone ever walks in (unintentionally or not) on someone engaging in some very saucy pursuits with a hologram? Is the holodeck the Internet of the late 24th century, with all the problems that go along with it?
This always bugged be, but the Holodeck's purpose was really more for training and education it just seemed to be more used for entertainment. It's also aboard a military vessel where in such a situation maybe you do not have a reasonable expectaion of privacy in the thing? (Quark's holodecks had locks, but his were owned by him and rented by customers, a different situation entirely.)
But, all and all, yeah you'd think it'd be just curteousy to ring the bell first, but in Barclay's case Riker sought him out because of insubordination/not showing up for duty. So Riker proably had a "right" to barge in there.
Now, when Picard cock-blocked him by barging in on him and Minuet, that was another matter.
