I'm not certain of age for Harley because I got her from a pound. But she did have a noticable chest widening around 2.5 - 3 years. She put on about 5 lbs of muscle over those months. SHe had always been skinny until then.
Titan is 15 months now and is still thin. He is no where near as skinny as Harley had been, which I figure is because he is male. But I expect him to fill out next year.
Harley Girl: Flashy Fawn, docked n floppy rescued 10/14/08
Titan: Classic Brindle, docked n floppy born 12/27/10