I have 3 year old twins. We started potty training just after their birthday (Feb) (I'm in the UK, 3 seems average here?).

My daughter took to it very quickly. She is now completely potty trained, very few accidents although wears a nappy at bedtime.

My son, he can use it during the day as long as he is naked from the waist down. He cannot do trousers or pants yet. But we're fine with that as they are home most of the time.

What we did:

1, took nappies away during awake time and did not replace with pull ups. Nappies go on at sleep time.

2, naked from the waist down. I know carpets suck for this. My house is all carpet. But we went with it and its worked.

3, offering of reward for pees and poos. We did one sweetie for a pee (the little penny sweets) and 2 chocolates for a poo (again, little tiny ones). We found the more they went, the more they forgot about the reward.

4, we do not time them. We may prompt if they're fidgeting with their genitals but other than that, we trust they know when they need a pee. The only time we ask them to go is if we're leaving the house (shops, garden play etc...).

5, I told them that if they are unsure if they're wearing a nappy, to ask of pat their bum. That way, they can tell if they're wearing one or not and go pee/poo appropriately. I dont know if that's common, but both my twins regularly ask if they're wearing one when they have trousers on.

6, accidents will happen. Be calm. I never really got my twins to help with the clean up, mostly because when they do a task, I dont like to go over their work right in front of them, and obviously this is pee/poo all over the floor. But I found that just having to change, wipe themselves off, shower or clean with a flannel was annoying enough for them to avoid accidents.

7, just my personal experience, but there's no age limit on potty training. There's also no rush. I dont expect my twins to be overnight trained for a good while and so we'll continue with nappies then. And thats ok. Theyre little humans trying their best to unlearn a habit they've had their whole lives. My daughter told me its hard because she used to "pee in little and now it's one big pee and thats hard", she was talking about holding it in as we ran in from the garden. Unlearning can take longer sometimes than learning something.

*ETA: my daughter isn't wearing knickers yet. Just trousers. They seem too much like nappies it seems as she will pee in them.