What is the common age that a child catches chickenpox? My nephew is 13 and has not got them yet. Should we be concerned?

No. Most children have received a chicken pox vaccine by the time they are 15 months old, they now get a booster shot also. Did your nephew get his vaccinations? If so, it is unlikely he will ever get chicken pox. If he hasn't had his vaccination, i would contact his doctor to arrange for that shot! chicken pox can be a serious disease, causing pneumonia, and it can cause shingles when he is older.
There is None. Prior to universal immunizations, preschool and school were the most common. With immunizations over the past 30 years, the disease is disappearing, thank goodness. With proper immunization, there is no "routine" age. If your nephew is immunized, he should be safe. If not he should be immunized.
Depends on exposure. If your nephew is vaccinated, that may be why he has not had chickenpox. As more children are vaccinated, the disease is less common and fewer children get wild-type chickenpox. However, the disease gets more dangerous as a child gets over age 10 (there is a higher rate of complications). A child over 10 who has not yet had disease should be vaccinated.
Chicken pox. Hopefully, your nephew already had both doses of the chicken pox vaccine and will never have to go through the misery of that infection. The vaccine has been available since 1995 in the US, and longer in other countries;it's very effective. We now rarely see the frequently serious complications of chicken pox disease that we fought in the past! If kids don't get vaccinated, though, they are at risk.
Vaccinated? None of our children should get chickenpox because they all should have received the vaccine.