Did a TATL a few months back (Ft Lauderdale to Barcelona), will do my best to answer:
Yes, there are definitely very few children (like none on my last one, a tiny handful of teens, and limited early 20s folks, definitely an older crowd). Quieter? Mostly. It was a smaller ship (Vision), and often the bars were quite crowded since basically every day was a sea day. It was also hard to find venues without something going on in them sometimes. Seems like there was always trivia or something else happening nearly everywhere we went during the day.
Drink Packages - yes, but if you just wait for the right time they'll have BO1OHalf off, so it comes down to like $55/day per person for two people. It's very easy to keep up with it though. Remember you're on the boat all day. It's all the drinks you want, all the soda, all the water, etc. Glasses of wine with dinner, drinks throughout the day. It is nice to just know how much you're spending and not worry about what things cost.
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No other big surprises. I'd definitely suggest a larger boat if you can though. While we did just want to relax, we ran out of venues to explore very quickly, and meal options were pretty limited. The main dining room, I'm not sure if it was just because it was TATL, or the new post-Covid stat of things, was pretty mediocre most nights, and there were only a few restaurants on the smaller boat so those got old quick too. Windjammer was actually surprisingly decent.
Honestly though, the rates on TATL are usually great, and it is well worth exploring.