You could absolutely make a case about how there is more money to be made from the families with young children demographic. However, Disney has a chokehold on that market and it's silly to think that Universal could make more money by trying steal market share from Disney in the families with young children department. Universal advertises themselves as the park families should go to when the kids are "too old for Disney." The tagline they use in many of their commercials is "kids grow up, and we think vacations should too." By doing this, they have positioned themselves as the market share leader for families with older children or young adults and it has been very profitable for them.
Expanding their brand to try and pull in more families with young children is a fine idea (and they are already doing this by bringing in Nintendo). But, it is important not only to themselves as an organization, but also to their core consumer base that they continue to introduce new, thrilling, exciting, and immersive experiences that you simply cannot find at Disney. No matter what type of business you are in, being just like your competitor is not good for business. Universal needs to give consumers a reason to choose them over Disney and I think they are and have been doing this extremely well over the past few years.