All my kids have learned to use the potty. My Man and I taught them somewhere between 2.5 years (too young, IMO, but she was our first) and 3 years 4 months.
From what I have learned from Child Development, the baby needs to know the difference begotten "wet" and "Dry" needs to be able to remove and put back on their own pants (or use dresses if it's a girl) wants to learn to use the potty and has the muscle control.
Newer research has shown that most children don't have full bladder AND bowel control until after the 3rd birthday! A few do, but most don't. From what I've learned, if you are bringing the kid to the potty more than every 2-3 hours, YOU are trained, the baby is not. They should be telling YOU they have to go, with reminders from Mom or Dad, of course.
I also believe Pull Ups and disposable diapers lengthen the potty learning time. Kids NEED realize, "OMG, I pissed my pants, it's running down my leg. Now, it's cold and sticky and I smell. I just sat down and it made a puddle. There's piss all over me and everything else. I don't LIKE this!" IMO, that is only achievable with cloth training pants. Yeah, it's more work, but it pay off in the end. They have to realize when they wet themselves. And if they can't go 2-3 hours without peeing, they aren't ready to learn to use the potty. Their bladder muscles aren't strong or large enough yet.
We tried stickers with the first one. (she was only about 2 and a few months when we first tried.) She ended up putting them all over her newborn baby sister.
She also didn't realize the difference between simply sitting on the potty and using the potty
. She was just too young, so we gave up for a few months, and tried again, sans rewards, and eventually she learned. She had some accidents but it wasn't too bad.
My second one REFUSED to use the potty at 2.5, she refused at 2.75, she refused at 3 years old. I'd put panties on her, she's sit on the potty and angelically smile at me, and do nothing, then run into the dining room and piss all over the dining room carpet. Still angelically smiling at me. (Cutest kid ever, I swear those curls and that cherubic face kept her alive!) One day, when she was about 3 and about 3 months, she woke up and said, "I wanna go potty." She ran to the potty and went. She then insisted on wearing panties, I readied myself for a day of cleaning up baby pee, but she kept dry and used the potty a few more times. She balked at the diaper at naptime, but I insisted. She woke up dry and used the potty. In a few days, she had never wet herself and refused to wear a diaper to bed or nap. She NEVER wet the bed, ever! That was it. This kid did it herself.
My next one was a premie and did a lot of things late (except talk, damn that bald headed kid, who looked like she was 5 months old was talking in sentences at 12 months) She did NOT want to use the potty, either. I refused to battle with her. I just pinned her little cloth diapers on each morning and waited. There was a potty upstairs and one downstairs, and somewhere around 3 years and 4 months or so she SLOWLY SLOWLY learned to use the potty. She was a kind of sickly kid and had accidents when she had a fever (which was often) so it took a while for her to be totally potty trained. She actually learned to read before she was fully potty trained.
She also had a health issue which caused serious constipation (and breastfed kids aren't supposed to get constipated, she breastfed until she was 4) which made learning to poop on the potty difficult. She understood WHY people used the potty, but it took her a while to learn how to use the muscles properly.
She, too, eventually learned. I say don't stress about it. They all learn. WHEN they learn is variable. It really doesn't matter, though. They will all do it eventually.