I've sailed HAL (first cruise was on that line, and I was in my 30s), and they also have a mix of ages on shorter cruises, much like Celebrity. Why everyone is angry that younger people are being denigrated but then have no problem doing the same to older people and/or other cruise lines is beyond me.
I've never had a bad cruise. I don't care how old the other passengers are, and I don't care if they care how old I am.