Actually, even in the first cartoon, the gang have always been teenagers. Evidently, the series bible for "Scooby-Doo, Where Are You" lists their ages as 17 for Shaggy and Fred, 16 for Daphne, and 15 for Velma (Scooby is 7 years old). 

And I do agree that the romances could have worked had they been presented effectively. Unfortunately, Daphne's crush on Fred became her defining characteristic, while it took Fred way too long to realize Daphne had feelings for him, even if he did eventually reciprocate. Velma, meanwhile, just comes off as controlling and manipulative in her romantic relationship with Shaggy, which isn't going to endear her to anyone. Like I said, trying to change a bad habit of your significant other is one thing; however, it seemed like if Shaggy tried to make any decisions for himself, Velma would freak out. I'm no expert, but I'm pretty sure that's not how a healthy relationship works.

And all this is frustrating to me, because when all the romance stuff isn't at the forefront, the gang are fine as characters in this. But apparently, even Joe Ruby and Ken Spears don't like the series, claiming it was "too cynical." Uh huh. And having the gang star in a direct homage to "The Blair Witch Project" and having it end with the gang missing at best, killed at worst, is totally not a downer.