Every kid is completely different, so there is no real age that works for everyone. One of my daughters was fully trained at 15 months, another at 18 months, but the others were older. My son was not fully night trained till much, much older. I think he might have been around 6 before he stopped completely, and that was after major effort on my part.
I start getting them to sit on it at an early age, but don't make it a requirement. We had a Muppets book we used to read, and my youngest had a DVD, but again, we didn't use them all the time. Candy didn't work for my kids, toys as rewards didn't work, but feeding the piggy sure did! My kids were all about the money!!! I bought each of them a piggy bank, and let them feed the piggy a penny for #1 and a quarter for #2. Worked like a charm. I'm talking, it happened so fast I couldn't believe it. My easiest trainings happened when I did day and night training all at once. It was a ton of work, but for a much shorter time period. I would just get up several times a night and put them on the potty. Every night we went a little bit longer between trips, and before a week was out, they were trained. My son was like me though. I was a bed wetter for a long time when I was little. His bladder was very sensitive, and I struggled with that as a child too (still do if I am being honest). With him I really had to dig in and wake up every single hour on the hour for about a week. Then we started letting a couple hours pass. Within a couple weeks he wasn't wetting the bed anymore.
I was a tired momma, but found it was easier just to buckle down and do the work. It took less time and frustration in the long run. Again, this may not work on every child, but I trained 5 this way, and they all fell in line. I think with the nighttime thing, it doesn't matter if they are holding it all night or not. It is about training the body to realize it is time to get up when the bladder becomes too full. As with training the body to do anything, it takes repetition.
The one other suggestion I have is that if your child becomes sick at any time during training, it may just be best to wait until after they are better to continue training. It is the one time I am in agreement with giving up. I didn't like sending my kids mixed messages. I just found that when they were sick it was a bit traumatizing to train them in anything. I put off breaking my son from a bottle at 1 year old b/c he had severe sinus infections and was just miserable. It would have been really hard for him to not have the comfort of his bottle at that time. My daughter that was trained at 18 months had a slight constipation issue that made her regress a bit until she felt better, but we got right back on track when she did. My 15 month old was almost trained at 14 months when she became dehydrated (she would and still does REFUSE to eat or drink a thing when she gets sick b/c she is terrified of throwing up, so she would become dehydrated and we would have to hospitalize her to keep fluids in her.) That undid a bit of our training, but when she got better, she was right back at it. I can't remember why we started so early with her, but she wanted to do it, so we did! Good luck!