My 2 cents:
For a frame of reference, my wife and I are in our mid-forties. I've been cruising since I was 4 years old, and have been through cruising as a small child, a teenager, a newly-wed, a parent with small kids, and now a parent with teenagers. I've selected and enjoyed both 1st and 2nd seatings many cruises through the years and have also enjoyed open sitting on free-style cruises.
For cruises out of U.S. ports, I feel that first sitting is too early and that second sitting is too late. I think that the Mediterranean cruises push back the dining times at least a half an hour which is better.
If I had to pick a perfect time to dine, and when I'm on a free-style cruise line, I like to arrive at a restaurant at 7PM.
I've been on ships with 1st and 2nd seating times at: 5:30 & 7:30, 6 & 8, 6 & 8:30, 6:30 and 8:30, 6:45 & 8:45, etc., but never 7 & 9 which would be perfect for me.
There are almost always 2 shows. Sometimes the show for the 2nd sitting is BEFORE their dinner. You never have to worry about missing anything if you have the second sitting. My problem with late seating is: a) I tend not to wait and I end up eating something late afternoon poolside, and b)that leaving a dining room chuck full (again) at 10PM, makes me want to go right to bed. This combines to cause over-consumption and lack of energy for late-night activities.
Whereas if I have early seating, I can easily wait until dinner to eat, and although I leave the dining room feeling full at 8PM, I usually rebound and feel great to hit the activities all night.
Lately, on Caribbean itineraries, ships tend to leave ports by 5, which has you on board before then with plenty of time to get ready for dinner.
Take it for what it's worth.
NCL Gem,1/13 NY-Carib
NCL Jade,6/12,Venice-Greek Isles
NCL Dawn,6/11,Boston-Berm-GARDEN VILLA
RCL Grandeur,2/03, E.Carib
NCL Dreamward,4/97,Berm,1st w/kids
RCL Nordic Prince,4/88,S.Carib,Honeymoon
NCL Sunward II,3/84,Bahamas
NCL Sunward II,3/82,Bahamas
NCL Sunward II,4/79, Nassau/Berm
HA Rotterdam,4/78, E.Carib
RCL Song of Norway,4/77,Berm
Carnival Mardi Gras,4/73,E.Carib
Greek Lines TSS Olympia,4/71,Nassau,St.Thom,P.R.
Greek Lines TSS Olympia,4/70,Nassau,St.Thom,P.R.
Italia Line Leonardo da Vinci,4/69,Berm
ECL Bahama Star,3/66,Nassau,age4