there is a lot of controversy about this all over the internet, but i started to train my little girl from 6 months, just sitting her on the potty for 30 seconds every time we changed her nappy.
By a year she was dry during the day and by 19 months through the night as well. My main reason was the cost of nappies.
I never forced my child and always made it fun with toys and books around and praised her if she did or didn't do anything but she soon learnt to ask for the potty if she wanted a poo.
My friends who had children of similar ages did this as well with equally successful results. My little girl is 3 now and we have never had a problem using the toilet and has never had an accident since about 20 months.
Basically I would say the best time is a time when you are ready to start as there will be accidents along the way but starting with using the potty when changing the nappy to me seemed the most logical way.
Good luck and hope it goes well for you.