On our recent 4-day, it was strange - most of the folks were about our age. Few children, few young adults. But our DS said there were a lot of kids his age. We overheard some seniors saying how nice it was to have quite a few folks their age.
We guessed that different areas of the ship attract different age groups, and you have the illusion of traveling with plenty of contemporaries. The kids programs are super, they generally stay in that area (deck 5) or their pool, so we really saw very few of the hundreds of children that must have been on that ship!
So, I think you'll come back from the cruise and feel like there were a lot of folks your age, regardless of your age!