Rule of thumb: reptiles in general are not 'tame' or 'docile', they are -tollerant- of human contact.
There are of course exceptions and individual animals that indeed show docility towards their human care-taker.
A juvie iguana will be easier to 'mold'... but that rule is not chizled in stone. Adolescent iguanas are the more wild-card, while purchasing a young adult may be a safer bet, because chances are someone already tamed it, or at least got it used to human contact.
Of course, there is the likely possibility of animal abuse, which means you could end up with a problematic animal.
There are a few things you can do to raise your chances of taming an iguana:
>buy/build the best enclosure u can afford. If you cannot, wait another year or a few more months 'til you do. A good, safe, functional, clean, and spacious enclosure that simulates and stimulates natural behavior is the most important element in iguana husbandry.
>do not rush. Many people expect the iguana (or any reptile) to tame down within days or weeks. Usually it takes months to even a couple of years before the animal completely trusts you (that alone is a monolithic step, considering we're talking about a reptile).
>positive reinforcement. Everytime the iguana does something you want it to do, reward it.
>read. The more you read, the better off you'll be when the time comes of getting your own iguana.
>ask. This is a great forum, and people here are always willing to help and answer to all/most of the questions. Granted, it is not as busy as it used to be, bu it still a magnificent source of information. Read back a few pages, look for topics or issues that may interest you, and save them for future reference. Remember, there are no stupid questions.
Good luck,
-IJ