The first two movies of the rebooted Bond-franchise were at the start of Bond's career (when he was likely in his late twenties/thirties), but when does Skyfall take place in relation to other things going on with Bond?
Timelines and James Bond don't go hand in hand, he's an ageless character and will always continue to be up to date regardless of when the stories are set.
Having said that if you're determined to answer the question, there are a few clues we can divine.
- He has acquired the Aston Martin, therefore it must be after Casino Royal
- The Aston has an ejector seat and machine guns which were fitted for GoldFinger
- It's Judi Dench's M character final mission therefore it must be after all of the Brosnan Bonds
- Q mentions the exploding pen which was last seen in GoldenEye
If you put these facts together Bond has most likely completed the missions in all the other films, however as I suggested at the start Bond and chronology questions are always going to be very hard because of the nature of the franchise.
For example in GoldenEye M is the new head of MI6 and meets an experienced 007, in Casino Royale that same M promotes him to the 00 section!
First of all, the James Bond movies (or rather the pre-Craig movies) don't adhere to an actual realistic timeline that goes through all of the movies. Without some small exceptions (Bond's wife and her death) there aren't any major cross-movie story developments and any Bond movie can be seen as rather stand-alone, even if some small comments and allusions to previous movies are made here and there (e.g. at the scale of Brosnan making a small remark about the jet-pack from Thunderball, but nothing serious or story-relevant).
But this is slightly different for the Craig-movies. First of all they're actually a reboot. Whereas all the previous movies didn't really care that much about their respective prequels, Casino Royale was the first to actively reboot the whole franchise and introduce James Bond as a completely new character recently promoted to 00-status. This means that any previous movies, in contrast to merely being ignored, don't actually exist in the world of this rebooted franchise.
On the other hand this new rebooted franchise comes with a much tighter cross-movie development and the individual movies' stories are interweaved much more tightly (especially noticable with Casino Royale and Quantum of Solace, though less with Skyfall). So while it is rather pointless to come up with an actual timeline for the pre-Craig movies, the Craig-franchise indeed has a sequential timeline with the movies happening in the order they came out and roughly at the timescales they are released. It has to be seen how far this is kept on with future Craig-movies, but since rather important story developments happened to major characters in Skyfall (especially M), I'm pretty sure the next movie will not just ignore those but rather reference them, be it only slightly.
So to answer your actual question, Skyfall likely takes place a couple of years after Casino Royale and Quantum of Solace and any other Bond adventures known from other movies didn't actually happen in this rebooted universe.
There is some continuity between films. It is best seen in some of the supporting characters.
It is complicated because the early films were made in the roughly reverse order that the books were written. So for example Quarrel is first met in the early book Live and Let Die and then dies in Dr No. For the film Live and Let Die they called him Quarrel Jr.
Felix Leiter's character is similarly distorted but there is some continuity within the books and within the films but never between the books and the films.
The Craig era films are best considered a reboot as they would have had to set Casino Royale in the 1950s to preserve continuity with the older films.
The Craig series takes place before all the other movies. In Casino Royale, Bond first gets his 00 license. At the end of Skyfall, the new MI6 office is identical to the one from the old movies and Moneypenny begins working as M's liaison. It's all in the last 60 seconds of the movie so it's easy to miss.