Laura:
Whether your baby is 8- or 18-months-old, you'll probably be using the stroller for most of her transportation and very little walking, so I would let feeding schedules, nursing options, and nap/bed times dictate a portion of your decision along with the temperaments of your older children.
I truly think any trip can be age-appropriate, but in my opinion, waiting another year will not make a large impact on your older children and their reaction to the many Disney characters they will meet, however you will find that you'll get responses from a toddler over a baby that are worth every dime you'd spend on
Memory Maker. Additionally, keep in mind that summer is hot and children get cranky in the heat. Would your crew do better with another year to learn how to cope in higher temperatures? Or do you think an 8-month-old will sleep through much of it compared to when she gets toddler-age?
If you have been blessed with a baby who can "sleep through anything," then a trip this summer can not only be considered but enjoyed. But if rest and naps are needed, you might want to wait until she's older so she can enjoy the night-time parades and fireworks.
Wishing y'all a magical vacation!
Whitney