I'll generalise about them, but some companies are obviously better than others.
I'd say mostly 40+, & I've never seen a child on one.
The cabins are just like an hotel room - but better designed. You visit a lot of places, but only unpack once.
Usually 120-180 passengers, & enough space to be sociable or reclusive if you wish.
Food is usually terrific, emphasis on fresh, local produce, & always options for dietary needs. Usually buffet for breakfast & lunch, a la carte for dinner. Drinks are often included, or sometimes a package is extra.
You're off the boat every day on an excursion - usually included in the price.
The excursions are always to interesting places, perhaps a guided walk around a town or village, an artisan olive oil producer, a famous cathedral, a stunning display of bareback horsemanship etc
Evening entertainment is very low key, which suits me. Often an on-board pianist, perhaps a fun quiz, & usually a local folk song and/or dance group.
Or sit in your room & watch a film.
I'm sounding like an advert !