I travel alone with my DS9 yearly to WDW and last year at 8 was when he really started being able to handle long park hours and was starting to outpace me on many of the days. When we first started going, he was only 4 and I only did 1 late night in our 10 day trip. At age 5 & 6, I upped it to 2 night. At age 7, he was good with another night added and at age 8, he was going from morning until midnight on a few of our days (we did MNSSHP) and we had no issues with fatigue, sore feet, etc... We do generally have 2 or 3 TS meals a day though and that does ensure we are getting regular rest/cool down throughout the day and I think that makes a huge difference.