Why is the dad so old?

Like really, he seems to be around 70 and the two younger daughters seem to be around 7 and 11. The oldest daughter (don't know how old she's SUPPOSED to be) looks to be in her late 20s, early 30s. Why such an age gap b/the oldest and youngest daughter and the dad and ALL of the kids? He seems as though he could be their grand dad ESPECIALLY the younger girls.