Regardless of age, I think people have very different abilities to do travel. I remember how much easier it was in my younger years to be able to roll with it, how much more energy I had and my body not hurting if I slept wrong on any given night! I think the same is true for kids. Different personalities mean different ages are easier. My kids are now 11 and 13 and we are just going on our first European summer vacation next June! But they have gone to Vancouver, Montreal, a ton of domestic locations, Mexico, Caribbeans. So it's not like they've never flown or stayed in a hotel room but my little one is very very particular with what she needs for good rest, and with opposite genders, it's hard to ask my kids to split a bed. So it's expensive unless we do Airbnb/VRBO. We started with Mexico when the youngest was age 5 and have taken a ton of road trips since then as well. I can't say my kids are "hard" to travel with but I also know where we are going to have an easier time of it - I'm not going to drag them to a place where it's going to be hard for them and for us logistically. So they are easy to travel with because I take care to take them on the vacations we are going to enjoy.
Ultimately however, I agree with the premise that if you have kids, it's not really a vacation, it's a trip.
No matter how well you get along with your kids and love them, for sure, you can't take them and expect it's going to work out in a high maintenance situation like a really nice fancy restaurant, do an Italian wine tour or take them to a hotel where it's so nice hey it's adults only! LOL
I can't imagine travelling internationally with kids under age 5 - between the gear and nap times - not sure how people are enjoying themselves!!! Unless you brought your nanny with you, what would you actually do, I mean even if you did camping, it's not like they can walk that far?!