As an answer to the OP, you know your children best. I don't think 5 years old is too old for a stroller at WDW, however, he might be too big to fit in most store bought models comfortably and probably over the weight limit (many are 35-40 lbs). I would say to definitely bring a comfy stroller for your 2 year old (the disney ones are hard plastic) and rent a single for your 5 year old. My kids will be 4 1/2 and 2 years old on our next trip and I am bringing a single stroller for each, but only because I spent a lot of money last year on a Maclaren stroller to fit my daughter who is on the tall side. If I didn't have that stroller, I'd probably rent for her and bring one for my 2 yr old. The cheaper models seem to only work for smaller kids, she was dragging her feet in most of them even last year, and a my double is too bulky and hard to push for a whole day. The singles worked out great since sometimes we split up.
Don't worry about what other people think or say. Those little legs take two steps to every one of ours and I know that by the end of the day I feel like I can't take one more step. I can't imagine how the little ones feel. Have a great trip!