I think everyone is answering this through the filter of their own kids and what they can handle. This varies wildly from kid to kid.
I have 3 kids. The first two were mellow, mellow, mellow. I could take them anywhere at anytime to do anything. We did loads of road traveling from the time they were born, but I don't think we did international flights until age 10. I remember the stewardess seeing us walk into first class with two kids and grimacing. I also remember the crew thanking us for having such wonderful kids when we arrived at our destination. It didn't even occur to me we would have any issues because these kids just rolled with everything. Once we reached that magical age of 10 we took them to so many places.
Now child number 3 is another story. He doesn't have a laid back bone in his body. He doesn't stop moving from the time he gets up until the time he drops into bed at night. We have only done limited travel with him because it wouldn't be fair to him and would be miserable to for us and everyone else around us. So those memories are just going to have to wait. There was a family wedding last year and I waved goodbye to my hubs and kids as they flew off to have a great time with all our loved ones. It gutted me to miss out, but there was no way I was going to subject my little guy to a cross country trip and time change. Sure enough, there were flight delays all over the place. It would have been a nightmare with a toddler. He's 3.5 now and can easily do a 5 to 8 hour road trip which is a big improvement for him. I actually had a lovely 2 hour uninterrupted lunch at a restaurant this week with him sitting right next to me at the table, so we are getting there. There is another family wedding in a few months on the other side of the country and my husband insists on flying him out. I completely disagree, but I am probably going to lose this one. I hate putting kids in a position to fail, and this is a big one for me.
I think at age 5 most kids are probably equipped with enough skills and training by parents to be fine travelers for many things. But I don't think (for us) international travel is worth it before age 10. I think I can take mine to many inspirational, different and cultural places domestically until he's that age. Have you seen how big the US is?