In a sense, this isn't an entirely fair question. Back in 1964, the Doctor was a mysterious alien about whom anyone knew precisely both jack and squat about. There was absolutely no reason for any of us to suspect that he came from a species that could live for thousands of years and changed appearance every couple hundred odd. That goes for the life expectancy of his granddaughter, Susan, too. Then, a few years later, the Doctor falls down in the TARDIS and gets a new face and personality, only to casually mention a year later that that he's already 245 years old. No biggie. So, let's look at the implications. In Marco Polo, it's stated pretty definitively that Susan is 16 years old. To be somewhat generous about the amount of time that passes before she leaves the Doctor, let's reasonably assume that she's 18 when she marries David. Surely it's only going to be a matter of decades before she and David have to have a serious conversation about why she still has the body of an 18 year old? After all, what would happen if she were to to trip over a rock and suddenly become a saucy redhead, for example? The different spin off works have tried to explain this away a number of ways. Legacy of the Daleks, for example, amusingly claims that Susan develops an elaborate movie make up skill set to make herself seem older. Others say she wasn't really the Doctor's granddaughter after all, so none of it even matters. So there you go.