you will find an ecclectic mix of people on just about any line.
3-5 day cruises tend to have more younger, single and young families.
10+ day cruises tend to attract an older crowd
There will be more families with kids on board during spring break-this seems true when it comes to Royal Caribbean and Carnival
Some lines like Princess do limit the number of kids allowed per voyage
I would look into Princess, Royal Caribbean and Carnival. Its almost impossible to compare line to line like comparing apples to oranges.