Missing teeth is medically known as tooth agenesis, there are many reasons why the teeth may be missing. Younger people may have missing teeth for sometime as the second adult teeth are growing through the gums and as most children experience tooth loss at around about the same age it is far less of a problem to them.
Missing teeth in an adult can be more of a problem and caused from various reasons, a tooth /teeth could have been lost from an accident, through tooth decay or gum disease, or even from overcrowding of the mouth so teeth have been removed.
Adults with missing teeth usually feel self conscious about their mouth and smile. Today there are a few treatments to replace gaps, as well as dentures there are natural looking tooth implants and bridges, if you have missing teeth there may be more than one solution so ask for advice and options from your dentist.