I agree with Hammer whilst taking Freddy's observation into account, but disagree on another level entirely.
I think that the prominence of stock teenagers in horror films was a mostly benign thing in the 80s, which has grown worse over time. However, it's not because trying to make partying teenagers sympathetic is a bad thing (I'd argue it's a step in the right direction for horror, giving us characters we care about and fear for), it's because they
ostensibly attempt at making the characters sympathetic, but in all reality they can't do it worth a shit.
There are about four levels of slasher movie character depth I care to get into; the nonexistent, the cliche, the pretentious, and the well-done. The "well-done" is good 99% of the time, so long as you care about and fear for the characters. The "nonexistent" is also OK; characterization in the original Texas Chainsaw Massacre was nonexistent, yet you could see the characters were human and thus still care about them.
The "cliche" is what popped up in the benign 80s, wherein characters were given about one defining mental trait. It's a step down, in my opinion, something that makes characters and consequentially plots dull. Still, it can allow you to root for the cultural-group you identify with (I root for nerds, which usually doesn't serve me well

), and find at least some characters sympathetic. Most of the time, the cliche characters are at least distinct from one another.
Today's problem is that writers attempt to overexplain characters, but do so in half-assed ways that make them seem like unreal stockpiles of sympathy or disdain. The main girl, of course, will be the sacred cow with
soooooooo many complexities in her life that are bioengineered to make target audiences identify with her; meanwhile, all the other important characters are given little personalities that define them as well, but it's basically just shit to try and explain to the target audience why they "had to die."
It verges on sickening.
It's pretentious, trying so hard to evoke sympathy or hatred in such artificial ways; I
hate the word "pretentious" for how often it is used to dismiss art and analysis, but in this case it's very appropriate. Filmmakers create such characters with the pretense that copy-pasting actions to characters with no regard for real personality will still yield a purely sympathetic or downright hateable or dismissible figure. But it doesn't work; it just brings conscious viewers disconnect from the obvious unreality.
So in conclusion, the problem lies not inherently with teenage characters, it just so happens that they are the ones who mostly fall prey to such shit characterization. It's the pretentious mindset of demographic-oriented writers that makes the "bad teens" act as though their "immorality" is the all-encompassing representative of their personalities. The same mindset that piles all the "good" traits on to one character, often bordering on Mary Sue territory.
Note that in these cases the "good" character is clearly going to live, whereas the other characters are typically not even teased at surviving. Hardly any attempt is made to get the audience attached to the latter, nor is any
good attempt made to get them to truly fear for the former. Compare to the other characterization styles...
Well-done: All characters are sympathetic, but tauntingly not "above harm."
Nonexistent: All characters are neutral, and in their neutrality sympathetic. Anyone can die.
Cliche: Some characters are more sympathetic than others, and more of less likely to survive. Yet, on the surface, morality is typically "neutral" and allows for sympathy even for those who are clearly gonna die.
...and the reason why characters in modern horror are so annoying becomes clear.
âThe Fright Night remake is a film which taps into the audienceâs deepest rooted fears, such as those of vampires throwing motorcycles at them. I dread the thought of a vampire throwing a refrigerator or a deskjet printer or... Iâd better stop before I give myself nightmaresâ