The actors playing the parents are too young

You answered your own question. The actors were simply playing characters who are older than they actually are. It happens all the time. Sean Connery is only 12 years older than Harrison Ford, yet the former played the latter's father.

It's possible that the actors were just playing characters slightly older than themselves, and I think that's acceptable, as long as there's not too large a difference. Matt Frewer might have been a bit young but pulled it off because he looked older than his actual age.

It's also possible that the characters had their kids at relatively young ages. I don't think any of the Szalinskis' ages were mentioned in the first movie, but for some reason I actually still have the novelization; in the book, Diane is in her late thirties and Amy is 15. In the first season of the TV show, Diane is 37 and Amy is 15. (The actresses on the TV show are only 16 years apart in real life.) Wayne's age isn't explicitly mentioned in the show, but he seems to be about the same age as Diane.

That was probably way too much analysis. =)

I'm surprised Matt Frewer was only 31 when this was filmed, he looked so much freaking older than everyone else, lol. Not just with his slightly receding hairline, but his face too. Yeah, Wayne & Diane were probably in their late 30s, so they would've been about 22 when they had Amy and mid 20s when they had Nick. That's totally normal, even if a bit on the younger side.

I agree with the OP though. Even if biologically speaking, you can have a kid at 16, 14 and even 12 of course, socially speaking that's not a parental age difference, its still like a much older sibling or cousin.

You should check out Poltergeist. In the movie, the father gives the ages of all of the family but it is unrealistic. I think it would mean the woman was 14 or something (I forgot).
In your case, you are comparing the actual actor's ages!

It's crazy, like they're playing the parts of different people or something!

When you're 17 a cow can seem dangerous and forbidden...am I alone here?

It works because the actors don't look young. Even though I'm older now, Kristine Sutherland just doesn't look 34 in this movie. She looks nearly 40. That's a testament to the casting, her acting, make-up and wardrobe. At that time most women in their 30s, especially mothers, just didn't look as youthful as they do now anyway. All the cosmetic things that a lot of people routinely do now weren't common in the middle class. Now this "youthfulness" is more borderline looking immature.