I think it is like a lot of things, you don't wait until you would expect them to do it and teach it then, you teach it from a younger age so that they get practice and become proficient by the time you would expect it.
For example, I taught my children simple niceties like please, thank you, sorry as soon as they were able make sounds. I didn't expect them to be able to say it consistently, but if they could make a 's' noise, they could say sorry, IYSWIM.
DD1 (4.7) has SN and her behaviour can be shocking. She needs 1:1 in group situations, etc. But her manners are impeccable. She says 'thank you' instinctively - she even said it to the nurse who gave her the MMR, through her tears (the nurse did point out that she didn't have to say thank you to someone who hurt her!). I don't think she is going to get the interrupting thing for quite some time due to her disability.
DD2 is 3.0, and she is starting to wait nicely at times.
DD3 is 16 months and can sign (& is starting to be able to say) 'please', thank you, and 'sorry'. She is getting quite reliable on 'please' and 'thank you', sorry is a matter of the will 
Anyway, I digress. I think that it is good to gently point out that someone is already talking, and it isn't their turn yet from a young age, but only really clamp down on it when they can understand.