I don't think 25wks is too early - I'm guessing you're probably about to wean, if not already, and the dental service will want to start giving you advice about preventing problems in later life - so why not start now? FWIW, I registered C and myself at the same time, probably when he was about 4months, and they were really keen to start seeing him as early as we wanted - partly for the above reasons and partly to get him used to being there and the atmosphere.
The only thing I was recommended not to do was take him if I was having anything more than a checkup, just so he didn't have to sit thro all the noises and stuff. He'll be going back when I go just before he's a year old. I think it's fab!