1 year, 27 inches. (Mostly tail)
This is, of course, in general, and is snout-to-tip-of-tail. I get this from Anapsid.org, below. There is no better resource, except Green Iguanas For Dummies, which has the same info.
Many small iguanas have serious intestinal parasites which are, fortunately, treatable. Others have poor diets, improper UVB, and/or poor habitats. Still others' owners' think their ig's age is older than it really is.
Small iguanas need to see an experienced reptile vet. Also, many smalls are healthy and live a normal life-span. The good news is iguanas can do catch-up growth.