Peaches are non-climateric, which means they do not get any sweeter after picking. They do soften & become more juicy as they age after picking though. So sweetness is 1 thing & "soft & juicy" is another.
Ideally you would pick them ripe off the tree for maximum sweetness, but all supermarket peaches are picked hard. They can still be sweet, though. I find the imported US ones typically are & the Ontario 1s are not. They are peachy but not sugary.