(dragons are measured from the tips of their headknobs. The image shows the difference between a gold and a green's growth, and the blue between the two lines for all the ages shows the possible sizes in between for all the colors... not perfect, but gives you guys a rough idea.)

Dragonets have little size variation when hatched, though queens tend to be slightly bigger upon hatching. As they grow, the size differences diminish up until the 3rd or 4th sevenday, in which the golds, bronzes, and many of the browns shoot up in height. Blues and greens grow more gradually.
At the 2nd month, the colors are obviously becoming more segregated in size. By the 4th month there will be an obvious gap between the size possibilities between the colors. Golds will be much larger than greens at this stage.
At 6 months, the dragonets will be 'locked' into a certain size constraint, meaning they will be roughly 60 percent of their adult size. They will grow more evenly at this point and time, and it becomes clearer who will be the larger of their colors. If a dragonet is larger or smaller than the typical color constraints at this age, they are very likely to stay within those parameters when they reach full size.
Practicing gliding and short flights during play are slowly encouraged in the 4th month, growing more vigorous until the 5th month comes around and the dragonets are allowed to hunt for themselves. Coming up to the 6th month, manned flight begins, for the smallest of greens are generally large enough to support even the heaviest of Weyrlings at this age. After a successful flight, weyrlings then move to personal weyrs for their dragons are practically spilling out of their couches by now.
From then, growth continues till they hit a turn of age, and some dragons may hit sexual maturity at this age.
Golds do not reach full size until the full 18 months, and some will continue to 'fill out' into their 2nd turn. Most golds are sexually mature at two and a half turns of age, though some have been known not to become sexually mature until 3 turns and some may not rise till 4 turns of age if certain conditions delay rising.
Almost all bronzes are physically capable of participating in a mating flight at 1 turn of age, though it is greatly discouraged. Bronzes typically reach full size at 16 months, earlier than most of his siblings.
Browns mature more slowly, usually only becoming sexually mature at 13-16 months of age.
Blues vary greatly, some being flight-ready when the first green hits the sky, and others needing to hit nearly their 2nd turn before they show any keen interest in females.
All greens are sexually mature at one turn to 14 months of age, though some may not rise till nearly 3 turns of age if their rider is immature.
Barring golds and bronzes, all dragons tend to stop growing at 18 months (turn and a half)
((Some of these notes are just something I added here and there, like with the blues, but from what I've read in the books, bronzes mature quickly, golds mature slowly, and browns mature more slowly than bronzes. Technically this would mean blues and greens would as well, but greens wouldn't mature so slowly because they can rise so early and so often. So.. that's how I figured all this out. =P I'll have to make some changes to the Weyrling info since I made the Weyrling's first flight and finding a Weyr a bit earlier. This is because the dragons I designed were smaller when I made those rules up, but with the new dragon sizes it makes more sense for them to be out and about before then.))
** Also, with dragons reaching sexual maturity at a turn of age, I'm thinking that it is possible for Weyrlings to be able to graduate when their dragons are at a turn of age. A dragon's sexual maturity doesn't matter at that point because at that age, the dragonrider lifestyle both sexual and non would have no negative effect on a 12 month old dragon, even if they had not experienced their first flight. This would also make more sense because a Weyr wishes to replenish their wings quickly, and if most are mature and flight-ready at 12 months.. why wait the extra 6 months? What do you guys think?