Aging Clark and even Lois up a few years won't hurt them. Having Jon, naturally, go from 10 or 11 to 17-18 isn't THAT big of an age up for "30 something" people. Hell, they could STILL be "30 something" at the end of it too.
What Jon (and other fans of young heroes) want, is to simply see these characters grow like Dick and his generation did. But that's something that really hasn't happened SINCE their generation. We either get artificial age ups (like with Jon), age that keeps jumping around (Raven and Beast Boy), or stuck with Ash Ketchum syndrome (forever one age, like Tim). Jason Todd was probably the last one (with Kid Devil I presume, both are in that weird in-between era) to actually grow up, if off-screen.