In general, Royal Caribbean attracts a younger passenger than does Celebrity. Also, the longer the cruise, the older the passenger.
There is never a guarantee whether any cruise will have single people. Deciding between the two cruise lines, ships and itineraries should depend on a variety of factors, just in case neither of those cruises has single people, or in case they both do. Celebrity in general, and the Constellation in particular, are both probably more upscale than Royal Caribbean and Grandeur of the Seas.