None of the kids are old enough to where "aging out" would be a genuine concern. The show might've had to change as they became teens, but they're throwing in the towel before reaching that point. Family sitcoms have had no problem accommodating that in the past. 

I'd guess diminishing returns or Rath being an actual mother of two young kids herself are bigger issues.

BowmanvilleBob wrote:

The show's creator indicated last May that the fourth season would be the last one, and that he had written a finale that tied together all the loose ends for the whole family. As he points out, the show is simply getting too expensive to produce. Ratings on The CW haven't exactly been explosive, if this snapshot is to be believed.

Not too surprising. A lot of shows end up that way. The leads and the key crew members have fixed salaries for XYZ seasons. That contract goes up for renewal and they're entitled to raises. Suddenly the network has to wonder if funding season 5 of a show makes more sense than a new series all together. This is an era where it's very rare for scripted shows to grow in viewership (less true for episodic sitcoms, but they aren't immune) through their run, so sometimes putting that money elsewhere makes more sense.

I'm guessing the official announcement only came now because CTV was hoping season 4 might do better than it has or that The CW might be willing to pick up more of the tab.