I would consider "early" potty training to be in the 18-month to 2 1/4-year old range. "Late" potty training is 3+. Then, there's a big spread in the middle that would involve issues on both the early and late sides. I've chosen to train my children early (daughter at 23 months; boys--just begun at 25 months) and Carrie's gone with the later age range (3 1/2 years for her boys).
If you haven't gotten to the potty training stage yet with your child(ren), it's worth considering the following; there really isn't a right or a wrong way to potty train as long as you do it, expect your child to obey once he/she demonstrates full capability/knowledge, and keep the peace in your household :).
Pros of Early Training (these are all generalizations):
- less laundry/effort if your child is in cloth diapers--maybe a year and a half less!
- less money/trash if your child is in disposables
- child still wants to please Mommy/Daddy, so battles of the will aren't typical
- rewards can be smaller--a Skittle, an M&M, etc.
- you might not be out and about as much as when the child is older, so you'll have lots of home time to practice
- much more parental time/involvement required (child can't pull his/her pants up, child can't reach potty by him/herself, etc.)
- can take longer just because you're often starting with a child who doesn't have tons of awareness of his/her bodily functions
- typically easier, if you can find the right motivation for your child
- less parental involvement needed (child can dress him-/herself, reach the potty, etc.)
- typically faster--the child is usually pretty aware of what's going on
- can result in a battle of the will; parent's approval isn't quite the be all, end all it is for a toddler
- you have to spend that much more time/money on diapers!
- you might be move involved in activities (like preschool) that can complicate time needed for practice/reliability