Before even looking in this thread, when I saw the title, I immediately thought 5.
My family is a little scewed since we have 4 girls ages 4, 6, 8, 10. On our last trip we rented a double stroller for the 3 and 5 (at the time) year olds. Every so often the 8 year old would sit in there and we'd have to tell her to get up. So as you can see even 8 year old will get tired towards the end of the day (understandable.)
A stroller can be a hindrance, but it can also be a helpful item at the parks. For example, you can use it as cup holders or to store things while you're on rides (ponchos, things you've purchased.) I know they could be stolen (as well as your stroller) but we've never had anything taken from it. You can keep your sweatshirts and jackets there during the day and use them at night. Bla bla bla....