Why are parents leaving this late? Start about 2 yo for little ones who don't have developmental issues. Make it fun. Bribery is your friend. Nice weather in the garden. Take off pants. Boys will water your plants and love it. Girls might be fascinated watching a stream without nappies. Friends round with children further on or the same stage of toilet training. Lots of lovely flavoured drinks and see who can wee first etc and get chocolate, clapping, singing, cake, anything for doing the wee thing (or even a poo) in the potty or toilet. Coloured bubbles, sparkles or washing up liquid in toilet or potty. If you live without a garden take them to the park/woods without a nappy or just with light pants on and play games with lots of drinks.
Remember, if they can't feel or see the wee or poo coming out they won't get the message. Just make whatever happens fun and use some imagination/bribery but most of all praise.