See we have a bit of a deadline here re potty training - they can't start school until it's done. This obviously impacts on childcare costs. DD1 will be starting school at 3.6 but LO should start at 2.10 - I'm not too fussed if she has to wait a few more months and start in the January at 3.2, but I understand people wanting to get them into the school system asap and skip childcare costs. LO will start at a month younger than DD1 is now, and DD1 would have been ready to start in January but I chose to leave it til September and screw the costs as I wasn't sure where I would have been on the rtw issues. Disclaimer: school here at 2/3 is not like school there at 4/5. I'm not being a tiger mum :o
Re readiness for potty/loo training - I read an article a few weeks ago that one of the key indicators is the "Moi/je" vocabulary - once toddlers start to say me and I (my and mine comes later) the article said their self awareness was at a level that indicates they are ready. This is rarely before the age of 18 months which is about the earliest age they may become able to control their spincter. Another indicator is being able to walk up and downstairs (not crawl) as apparently that uses the same muscles as those which control the sphincter 
Not sure how much of that is true - but it was a viewpoint I had never seen before. It said that in the past mums would start training kids at 12mo but they would almost never be accident free or even go of their own accord to the loo before 18-22mo. It was v much parent-led, time consuming etc, but saved on nappies/washing nappies. It touched on elimination communication but was quite negative about it basically saying it was a lot of faff.