(A bit necrotic. But never mind. [And when I started this, there'd been no replies. Ah well.])
A) [Assuming you mean with real people, not just probes like Curiosity]
The 'when' and 'why' may be linked together quite a bit. There's reasons to not do, mostly "Money could be spent better", and there's reasons to do, which for Apollo was "We[1] need to beat the Russians!", but it's hard to properly evaluate what the overriding future "to do" reason will be that overrides all the "not to do" ones. If SpaceX, etc, get their act together, it may be "We can beat Virgin Galactic/Scaled Composites" on a private venture into manned space-flight, and this may even continue into intrasolar. Right now, I can't think of any national (i.e. representative national space agency, with the backing of the government) programme that has much of a definite target for manned transit to the moon. Well, there's the Chinese, by many accounts, but it's hard to say whether there target reaches (or stops at!) that distance of transit. They might be happy to just 'camp' in LEO for a while, once they've proven that their current programme is consistently capable of getting over that initial hurdle.
There's a whole lot of wiggle-room, though. Mainly for anything unforeseen happening. (Suddenly North Korea comes out of nowhere and actually sends a person to an asteroid. Or something requires a Deep Impact/Armageddon-style mission, because "Robots just can't do what's being asked of them".) Pessimistically, we're looking at inertia keeping us down on Earth (and even abandoning the foothold we currently have, in the ISS, if I'm feeling rather gloomy). Optimistically, someone'll find a very good reason to send foot-troops further out than they are now. The utter flip side, in the pessimistic direction, we find an absolutely vital reason to go out there (e.g. Bellus is coming, and Zyra will be 'our' new home), which at best gets some of us off-world before
B) We can now. Or know, at least, that if we can make (a tolerable) centrifuge then the passive effects of low gravity can be mitigated for at least off-duty hours. And we're currently learning about the dietary changes that might be useful to offset even the biological collateral from the remaining periods of freefall/microgravity/light-weightedness.
[1] The US, that is. I'm not USian. I'm UKian. We went "Yes, we've successfully launched our first satellite, let's cancel our space programme while we're on a high."