I think 20/30/40 years ago mums had a different definition of being dry. My mum claims I was potty trained by 18m and whilst I can believe I was out of nappies and going on a potty regularly I VERY much doubt I could take myself, at the correct time, WITHOUT prompting, pull down my knickers (and back up again) and wipe my bum at this age. I suspect she just knew my "cues" and took me regularly herself before I had an accident. Oh as well as "lifting" me at night.
With DD I decided to try and potty train in the summer when she was 2.5 years having convinced myself she was ready. All her friends were doing it and therefore so was she!!!!! It was an utter nightmare and we had accidents FOR MONTHS. Yes she was out of nappies and going on a potty, but in no way could she recognise the need to wee herself(which I think is a vital concept), and if I forgot to take her then it was a complete change of clothes. At the end of it we had to re carpet downstairs as I couldn't bear the thought of the amount of wee soaked in it. She finally got it properly by about 3 and was dry at night by about 4.5. She now has a huge bladder capacity, but it was hell at the start.
With DS, I decided to leave it as long as possible having been traumatised by his sister. However at exactly 2y3m he found a pair of big boys pants (that id bought previously)put them on and immediately refused to ever wear a nappy in the day again. There were a few accidents of course, but he is not yet 3 still and is completely dry, day AND night. He was just ready and it was soooo much easier.
Certainly don't worry about being dry at night. 10% of 5 year olds are not dry at night and 5% of 10 year olds, which is a lot I think! Apparently its a hormone that suddenly gets released and that can happen at any age.
My advice - sit back, don't think about it (but do have a potty available so they can get used to it and play) and one day they will suddenly get it and want to try.
oh- and stocking up on lots and lots of big packets nappies during some supermarket promotion will guarantee they then do what my son did and leave you with lots of unwanted nappies.