Sometimes there's partial aging. While PJ of the "Family Circus" will forever be a toddler, "Blondie's" Baby Dumpling grew from child to teenager -- and then stopped aging.
A few strips have their characters age, although at a slower rate than the real world. Rex Morgan, MD is a good example. When it premiered in 1948, Morgan was a newly-minted doctor in his early 30s. In the current continuity he's the father of a precocious 8-year old. But not as an old man in his late sixties, but as one in his early forties.
In a 9/28/14 Barney & Clyde sequence, Gene Weingarten, Dan Weingarten, and David Clark make plain a common ploy used in strips where aging is an issue..
That is to say, never provide specific birthdates for characters. Because anyone can do the math.