It is very difficult to recreate an accurate and lifelike image of someone from a picture. My bro.-in-law is an artist and has a master's degree, but he says human faces are the most difficult to draw for him. He does not do portraits very often, because he says the facial features can change depending on the mood of the person. I think that's something only an artist would notice.
Age progression pictures would be extremely difficult, but most of the ones I have seen are pretty accurate.
As for the teeth, that's one of the least recognizable features to me. And if they are trying to ID remains, they have to take into consideration that the person may or may not have had good dental hygiene, needed braces, had missing teeth or so many other things to take into consideration. They could guess at it, but not showing the teeth could make a difference in being able to ID someone from a recreation. Family members could think, well, that looks like my daughter/son/mother/brother, but the teeth are all wrong, so no, it's not them.