It is solely up to Her Majesty to consider an abdication or not. Posters who claim "She will never step down" are triggering me because they claim to know something no one knows. It is the same as the one poster claiming that corgi Bella is her favourite and the other claims that it is for sure corgi Max. The morale of the story: no one knows.
Also posters claiming that an abdication is impossible: du moment Her Majesty informs the Prime Minister about her intentions, the Governments of all her Realms will provide everything to fulfill Her Majesty's wish. It is most unlikely that Antigua and Barbuda, Tuvalu or New Zealand will oppose the Queen's possible intention.
I'm going to both agree and disagree with you (both here and in what you've said elsewhere on this thread).
I agree that it is entirely possible for HM to abdicate and if she so wished to her governments would make it happen. I don't think it would be as easy as you seem to think, as at least some of the realms if not all of them will have to pass their own legislature to allow for an abdication. It wouldn't be nearly as quick and (relatively) easy as the abdications we've seen in the Netherlands, Spain, Belgium, or the Vatican. While the Queen is well within her rights to abdicate because of her age, and if she so wishes then we should respect that, but there is already a system in place for if and when the monarch is unable to perform his or her duties, the Regency Act, which is far easier to put in place than an abdication - it only applies in one realm and requires no additional legislation.
I disagree with the idea that because abdications happened in these countries the Queen should happen. You're not considering the reasons behind these abdications, or the potential consequences of an abdication of the Queen.
- In the Netherlands there is a long history of monarchs abdicating. Beatrix choosing to abdicate has no potential backlash because it's something that is expected to occur there.
- In Belgium Albert has been involved in an ongoing paternity scandal. At the time of his abdication his (alleged) illegitimate daughter was pressing a court case against Albert's legitimate children to prove her paternity. The case was publicly dividing the family, and seriously threatened the popularity of the king. In abdicating he may have opened himself up to attack, but he may have also helped secure the monarchy (although, subsequently, his actions have done much to further weaken popular opinion of him, and made it seem like he doesn't respect the new king).
- In the Catholic Church, there is an ongoing and extremely serious issue of sex abuse within the church. Benedict was repeatedly accused of being involved in the cover up of the scandal, and two separate lawsuits were brought against him and others in the church for covering it up. Furthermore, Benedict took a stance on issues (birth control, HIV/AIDS, homosexuality, same-sex marriage, etc) that is extremely traditional and hasn't helped the church in retaining membership. While the official reason for his abdication may have been his health - the first pope in 700 years to feel his health prevented him from fulfilling his duties - in reality the reasoning was probably the scandal and the fact that he (or the church) realized that they couldn't go forward into the 21st century under Benedict's teachings. It's kind of important to note that Francis is taking a very different approach to these issues than his predecessor took.
- Juan Carlos may have officially abdicated for health reasons and because he doesn't want his son to grow old waiting like Charles - which is a load of BS, as Charles may be an old heir, but he's definitely not simply waiting around for the job - but once again, it's naive to think that that's the actual reason. While JC may have once been extremely popular, in recent years the Spanish monarchy has dropped in popularity by a lot, as has JC. He's been involved in several scandals, from the accusations of cheating, to his hunting, to his overspending (and all of that came from just one trip). His daughter and son-in-law are also involved in a very public legal case, where it looks like they're most likely not innocent. While support for the monarchy is at, or below, 50%, support for Felipe is much higher - and JC, a man who had to fight for his position, unlike many other modern royals, understands all too well the importance of remaining popular amongst the people. His abdication may be for "health" reasons on paper, but it's really in order to attempt to save the Spanish monarchy overall.
Three of the four monarchs who abdicated in the last couple years may have claimed health reasons, but they clearly did so because their role as monarch was jeopardizing their monarchies. The only one who can legitimately be seen as abdicating because of her age is Beatrix, and she belongs to a royal family were abdications are seen as the norm - it's been 124 years since the last time a Dutch monarch died in office.
QEII is not in a position remotely similar to that of her abdicating counterparts. Unlike Beatrix, she is not in a family that has a tradition of abdicating - quite the opposite, in fact. Abdications in Britain are things of scandal. Unlike Albert, Benedict, and JC her actions have not caused or associated her with any huge scandals. She is not a much disliked monarch, there are no massive calls for her to abdicate in favour of her son. There is no evidence that if she continues to remain Queen the monarchy will be destablized and in danger of being removed. In fact, it's rather the opposite in within the Commonwealth. In several of the realms, it is expected that when the Queen dies republican referendums will occur - and an abdication will more than likely aid such debate. The Queen remains one of the most popular members of the British royal family, which sadly cannot be said for her son and his wife. There is still call from people for the throne to skip over Charles in favour of William - despite the fact that Charles is one of the most active members of the royal family, while William remains at best a part time royal who has yet to really prove himself (not meant as a criticism of William). If the Queen were to abdicate, it could very well spark calls for Charles to abdicate.
The abdication of Beatrix was done knowing that it would have no negative affect on the Dutch monarchy. The abdications of Albert, Benedict, and JC were done with the expectation that things for the monarchy/church would be better under new a new monarch. The same cannot be said for QEII. Her abdication will not better the realm, nor will it have no negative affect on it - which is not meant as a criticism of Charles, as I think he'll be a good king when the time comes, but rather a critic of how people will react to an abdication.
If the time comes where the Queen feels she is no longer able to perform her role and somehow the requirements of the Regency Act aren't met (which is a stretch, as they're rather broad) or she doesn't wish to have a Regency and would rather abdicate, then I think we should respect her decision and support her in it. However, I honestly don't think that her abdicating will be what is best, as there are too many potential negative consequences. The Queen abdicating will only serve to destabilize the role of the monarch and monarchy.
Personally, from all that I've read about the Queen (which is a fair bit), I don't think she would ever abdicate unless doing so would be to the benefit of the monarchy. She grew up in a family that taught that abdication was selfish and to go against the plan that God had set out for you. She saw her Uncle David essentially being cast out from the family and exiled from his home for his decision to abdicate. The three most influential people in her childhood - her parents and her paternal grandmother - all had very strong feelings about abdication. As such, I don't think the Queen sees her role as being a job one can simply retire from, or the position as monarch (particularly a crowned and anointed one, which David was not) as being something you can just give up. If she thought her abdicating was necessary for the good of her realms, she would do it, but it isn't, so I don't think she will.
Instead, I think if the time comes where her health prevents her from being able to fulfill her duties, a regency will be established. Currently, the Regency Act allows for a regency to be established if "the Sovereign is by reason of infirmity of mind or body incapable for the time being of performing the royal functions". If the Queen's health becomes such that she can't perform her duties, she is of infirmity of body and can pass on her duties without having to abdicate and create any ensuing chaos.