Model release is nec if the photog wishes to use the pictures for commercial purposes. It is to protect the photog more than the photog, since the model release basically says what the photog is allowed to do with the photos and that the model has been paid for the photos and thus has no further claims on the photos.
If the photog wants to use the images strictly for his own portfolio (eg to display to clients who come to his studio), a model release is prob not necc. However, if the photog is to be put on the web or to be published in mags, etc., then a model release is needed.
Age is irrelevant to the purpose of the model release in the sense that regardless of the age of the model, you need a model release if you want to use the photos commercially. However, as it is basically a legal agreement between parties, a model release can only be properly granted by someone above 21.
TFCD does not necc mean that you need a model release per se. TFCD is an exchange of time for pix, so if the photog is satisfied with just using the photogs for private use (eg in personal portfolio) then it may not be necc. However, model release is common since the photog usually expect more than just portfolio shots out of TFCD (although I think that if the model is really new, the photos won't be very saleable).