I donated last Tuesday, was OK walking, but tried to run on Thursday and my legs and butt went on strike - I think they were chanting "Give back our myoglobin, or We Shall Not Be Moved". I usually expect a dip in exercise tolerance for a good week afterwards. Presumably, too, us littl'uns have a bit less blood to start with and experience a bigger percentage drop with each donation.
The fluid volume is made up again very quickly, but it takes time to generate new blood cells (though our bone marrow will pick up on the fact that we've had what is effectively a fair-sized haemorrhage and go into temporary overdrive) so our oxygen-carrying capacity is reduced for several weeks. We adapt before then, fortunately.
It's a good excuse to have red meat every day for a few weeks :-)