I have a 1 mm (1/2mm yr.ago) space between my front teeth. teeth shift later in life? if i do not have periodontal disease what could it otherwise be?

Anterior space: Teeth can shift slightly over time, young or old. The more ideal the occlusion (bite), the less likely the chance of that happening. Another possible cause are habits such as biting on pens, bruxism, etc. Your dentist can evaluate your bite, possible occlusal disharmonies, habits, etc.

They can shift: As we age, our entire body ages... You are not the same as you were 40 years ago. If you lost vertical dimension over time (due to grinding of your posterior teeth), your lower will hit into your maxillary teeth, pushing them forwards. Consult with your local dentist for an evaluation.

Orthodontics/retain: Orthodontic therapy can correct the ever increasing space, assuming you are otherwise in good dental health - no tooth decay or periodontal disease. Teeth can move at any age and be corrected at any age as well! if you have any kind of dental disease, correct that first and than fix the space! good zluck?!