I had Topher in at about 20 weeks and he's due again to go in next week. My groomer believes (and I agree) that the younger they are, the better (within reason, of course!) as they get used to the grooming process.
I didn't get much hair taken off, just a bit of a trim, but she bathed him and pampered him and did such a lovely job. He came out looking like a new man...er...yorkie!! lol So neat and handsome. Needless to say, they typical boy that he is, he didn't stay that way for very long!!
I basically took him there, though, so he could start getting used to her, and thankfully, he was very well behaved and he adored her.