My daughter was a similar age to yours when she stayed in the infant room this summer. For her, I knew there would be a move up in Sept. - and there were other kids around her age (one girl a few days younger), one a month younger - so I was actually happy with her being the oldest in the room and getting to experience that for a bit.
She's 16.5 mos now and will be moving to the toddler room.
My son is 4 - he will be the youngest in his class b/c he is going to kindergarten next year - I wanted him to experience being the youngest at our daycare/pre-school (Junior K class) - and then we can make sure he is ready for K next year.
In your shoes, I would probably want to know when they do plan on moving DD up.