I would definitely consider them a senior at 9. However, sometimes the individual dog may display some aging even before that. With a large breed, I consider them more of a senior around 7 or 8, depending on how they're doing. My small breed dog I considered a senior when he turned 10, although he didn't really slow down until he was 12. He's 14 now and really showing his age these days.
Xan Varik von Fernheim (born 4/3/13)