Spring break for the US is not one give date. It spreads over about a month and a half from early March to late April. Some schools have their breaks in the early parts and some schools have their breaks in the latter part. My friend was on the Golden on March 21st this year and it was over run with kids. Usually on Princess ships, you hardly even notice the kids. But on this cruise, kids were running up and down the hall, knocking on cabin doors and running away, leaving food in the elevators, pushing all the elevator buttons, etc. Also, going intot he adult pool unsupervised and staying there.
On a normal 7 day Princess cruise that I've been on, the average age of passengers is around 40, I would say. Some families with kids, some older folks, but average in the early 40s.
"Spring" break is usually the week before or the week after Easter. BUT it could be up to two weeks before or two weeks after. Very popular time for families to travel together. If your cruise is a longer one, that might help you a little.