No, Moff is on record as saying that his original intention had been to cast someone in his forties, but that Matt had been so good at the audition he'd changed his mind.

DW is supposed to be a family show, so to an extent, the age of the actors shoudn't be a factor. But when the two leads are so close in age, I think it is, because a part of the audience is going to be 'left out', if that makes any sense.