I think that starting them at a young age is a good idea. I've already started Gracie with bows and vests. However, I think a lot depends on their attitudes. Some just don't like clothes, some love 'em.
I didn't get Annie B until she was nearly 5 yrs old. She was a rescue and had no socializing or idea of how to be a dog. It didn't take long tho to discover her Diva side. She loved to dress up & she'd pose all day long for pictures. I do encouragement cards for my family and friends each month and last year was all about Annie. I did a different theme each month and the only time I had trouble was when I put her in her motorcycle gear and we didn't go for a ride after the photo shoot. She pouted for the rest of the day after that one.
Dang i miss that girl
Anyway all this to say, I think it's as important to have one that enjoys it as when you actually start to dress them.
1) April showers bring May flowers
2) My wild child
3) The Dog days of summer
4) Ready for back to school