older son potty trained at 20 months... yeah seriously... never looked back, 1 accident and he's 5
little brother almost 4 would gladly still be in dipes if it was up to him.. we have went back and forth from sure this is fun to no way will I pee in that thing! we have resorted to having a 'pee cup' in the bathroom, a fun plastic cup with a bear on it that he gets to pee in, he's okay to do #2 in the potty, just not pee, out and about in public he's okay to pee in other toilets, just not ours at the house... who knows??? he will wet the bed for a few days in a row then will hold it through the night for days, I keep a washable piddlepad under him at night just in case.
I don't know why they regress but eventually, hopefully one day our little guy will be dry all the time!