Tango didn't gain any more height or length after about 7 months of age. He weighed 2.5 pounds at that age. But he didn't gain his mature physique till he was close to 18 months. His chest filled out, his tummy tuck became more apparent, he got a little more fleshy around his head which caused his eyes to change shape a bit, giving him a more chi-typical bug-eyed appearance.
His coat changed too, a bit. He has a slight ruff around his neck that didn't become apparent till he was well over 12 months of age. He's also double-coated, which I didn't notice till he was over a year. He topped out at just under 4 pounds, so that weight increase occurred between the age of 7 and 18 months.