Yeah those 8 and 9 year olds really really really need those strollers... Not to mention that 12 year old whining about his feet hurting, sitting in the stroller while his 4 year old brother was walking around (and had been all day) just fine. That was my all time favorite. But nope, no correct age whatsoever... sooner or later I really hope they start making "big kid" strollers for the 10-19 age set. That'll be the most fun time ever for mom and dad.
That said, I think you are awesome for being considerate and asking for advice about strollers. I recognize them as a necessary evil for younger kids and for those I consider age appropriate for a stroller, I have no problem with them. You most definitely fit into that category, and I hope you have a wonderful trip.