The American Academy of Pediatrics says there is no set age when toilet training should start. They suggest that the right time depends on a child's psychological and physical development. Children between 18 and 24 months often start showing signs of being ready, but there are some children who may not be ready until 30 months or older. The American Academy of Pediatrics also advises parents that if their child resists strongly, they will be better off waiting for a while.
Don’t be discouraged if you have a head-strong 3 1/2 year old, because there are some things you can try. Take your child to the local store that sells underwear with characters on it. Visit the underwear aisle and explain that he is a big boy now, and big boys are very lucky because only they can wear big boy underwear. Tell your son that he is especially lucky because you brought him to Target, or whatever the store is that you chose, where they have lots of underwear to choose from. He can pick whatever he wants.... Batman, Power Rangers, Spiderman, Bob the Builder, etc.
When you get home, let him try on his new underpants. If he can stay dry for an entire hour, he can wear them all day. Be sure he uses the potty first, to help him stay dry for the full hour. At the end of that hour, remind him to use the toilet again, and try for another hour.
He can also help you create a sticker chart that you hang on the refrigerator. Each time he successfully uses the toilet, he can help you place a sticker on the chart. When he reaches ten stickers, he has earned a special treat, such as a trip to the movies, a small toy, lunch at a restaurant, etc.
Odds are that some of his friends are already trained. Try to encourage a play date where he'll see them use the toilet. That alone might be enough to spark his interest. By 3 1/2, when he decides he's ready he will probably catch on very quickly. He most likely does not have a bladder control issue, it’s just that he needs to decide for himself when he's ready.
When the weather is warm, take advantage of it as a great time to train, because you can be outdoors where it's easier to clean up accidents. Potty training is hard work but you have to be consistent.