Children should be considered for a teeth-straightening treatment as their teeth start to come in to determine if they can benefit from that treatment.  Invisalign would probably not be an appropriate course of action as they are typically for after all of your adult teeth have come in but your dentist will design a tailored treatment plan for your children.
Pediatric dentists like Dr. Sbenati, in particular, are well trained in creating safe and effective treatment plans for children while helping them with their dental fear and/or stress.
If you are an adult your teeth straightening adventure can start at any time, simply set up an appointment at your dental clinic if you have an underbite, overbite, crowded teeth, crossbite, or another teeth spacing issue.  If you want a long-lasting and safe teeth-straightening solution a dental orthodontist is your only solution.