My 8 year old is extremely healthy, very active, and loves running (we go on family walks all the time and he always has to run instead of walk).
That being said - at Disney, an 8 year old is NOT too old for a stroller. This is YOUR vacation and no one else's and if you think it will make a more pleasurable trip for everyone, then go for it!
My son is extremely fit and healthy, but Disney is exhausting for
everyone and little legs with developing muscles walking 5+ miles a day is not an option for all or even most kids. Sure, you can take a lot of breaks, but that's not a style for us.
We have a sit-and-stand (aka caboose) stroller for our youngest and our 8 year old will sit or stand on the back the few times he needs a break (he didn't ride in it last year, but it was a place for him to sit and rest when we stopped - he did ride it in a good amount of time when he was 7). This keeps him going at our pace and the whole family is happy and able to accomplish everything they want to do because we don't have to stop for frequent breaks.
Just keep in mind that renting a Disney stroller for an entire week could buy (or be a decent down payment) on a good quality stroller - you might want to look into one of the
stroller rentals offsite (much cheaper for the week and you get to leave the park with it and not unload all bags and children and still get to the hotel). We have the Joovy Caboose and love it. It has a bench seat in the back, so my oldest can sit there when we stop for snacks and we don't have to track down 4 open seats or tables.
People are judgmental everywhere - don't worry about what others think. You're paying A LOT of money for this vacation and my philosophy is to enjoy it any way you see fit. If that means a stroller for your child, why should anybody else care about their age? My youngest will be 6 this year, so we are opting to not take the stroller, but if he gets tired, he is our complainer, so we are budgeting the money to rent one every day if necessary.