No, screening does not save lives even for the higher risk older groups never mind the lower risk 40-50s.
The evidence I'm going to quote below addresses the broader question of whether screening works for any age group (which was my original question). But, since the evidence clearly shows that the risk of having breast cancer grows with age (older women have higher risk) I hope that simple logic will show that this answer will be even more relevant to the younger group the question now addresses.
I also need to make a statistical clarification before quoting what thorough reviews say, as careless language is one of the reason that the stats are often misunderstood. One key problem is the casual confusion of all-cause mortality with breast-cancer mortality. It is easy to assume that mammography (which is designed to screen for breast cancer) must reduce all cause mortality because it catches some cases of breast cancer. But many studies show a reduction in breast-cancer mortality but no reduction in overall mortality. Guess which number is used by advocates of screening?
The latest Cochrane review (published in 2011) is here. The review was designed to assess:
To assess the effect of screening for breast cancer with mammography on mortality and morbidity.
Here is one part of the conclusion relevant to the statistical clarification i made above (I've highlighted the important bits and I quote at length to show the different results from different trials):
Three trials with adequate randomisation did not show a significant reduction in breast cancer mortality at 13 years (relative risk (RR) 0.90, 95% confidence interval (CI) 0.79 to 1.02); four trials with suboptimal randomisation showed a significant reduction in breast cancer mortality with an RR of 0.75 (95% CI 0.67 to 0.83).
We found that breast cancer mortality was an unreliable outcome that was biased in favour of screening, mainly because of differential misclassification of cause of death. The trials with adequate randomisation did not find an effect of screening on cancer mortality, including breast cancer, after 10 years (RR 1.02, 95% CI 0.95 to 1.10) or on all-cause mortality after 13 years (RR 0.99, 95% CI 0.95 to 1.03).
The overall conclusion was (my emphasis again):
Screening is likely to reduce breast cancer mortality. As the effect was lowest in the adequately randomised trials, a reasonable estimate is a 15% reduction corresponding to an absolute risk reduction of 0.05%. Screening led to 30% overdiagnosis and overtreatment, or an absolute risk increase of 0.5%. This means that for every 2000 women invited for screening throughout 10 years, one will have her life prolonged and 10 healthy women, who would not have been diagnosed if there had not been screening, will be treated unnecessarily. Furthermore, more than 200 women will experience important psychological distress for many months because of false positive findings. It is thus not clear whether screening does more good than harm. Note that more recent large scale studies are now questioning whether the apparent reduction in cancer mortality is real, weakening even further the argument that screening is useful (see latest update for references).
The review recognizes that it is not up to the medical community to recommend blanket screening as only the informed patient can weigh up the cost against the benefit.
Two key points are worth repeating. Screening isn't costless and the cost of false positives and unnecessary treatment are high. And there is no good evidence that overall mortality is reduced by screening. The intuition expressed by many (especially those lobbyists and politicians who backed away from the original recommendations of the Preventative Services Task Force) is not backed by the hard evidence.
This answers both my original question "does screening reduce mortality" and the more specific question now asked about screening in the low risk 40-50 year olds.
Some additional information to clarify some misunderstandings raised in comments
What is a Cochrane review?
One thread of comment on the original answer accused me of selectively quoting a result to bolster my preferred point of view. But what I had quoted was not a single result but a Cochrane Review. It appears that not everyone knows what that is and why it is the opposite of a single literature result.
Cochrane Reviews are systematic reviews of the complete literature of studies relevant to a particular claim. They review the known results for quality, bias and the strength of their results and produce a systematic analysis of the weight of evidence designed to summarise as best as possible the known literature. Wikipedia says:
The Collaboration [The Cochrane Collaboration is the independent organisation that conducts Cochrane reviews] aims to provide compiled scientific evidence to aid well informed health care decisions. It conducts Systematic reviews of randomized controlled trials of health care interventions and tries to disseminate the results and conclusions derived from them.
The reviews are described on the organisation's web site as (my emphasis):
Cochrane Reviews are systematic reviews of primary research in human health care and health policy, and are internationally recognised as the highest standard in evidence-based health care. They investigate the effects of interventions for prevention, treatment and rehabilitation. They also assess the accuracy of a diagnostic test for a given condition in a specific patient group and setting. They are published online in The Cochrane Library.
So when I quote results from the Cochrane Library I am not being selective, I am trying to reference the best possible analysis of the whole literature. Moreover, the Cochrane reviews should be less subject to bias than the reviews by government bodies who face political pressure and embarrassment when they change their mind on anything.
Why is this a controversial issue?
It is easy to misunderstand the risk-reward pattern of medical screening. Popular understanding tends to assume (in the absence of epidemiological evidence) that screening is cost-free. It's not exactly surgery, is it? This view has been reinforced by the advice produced by governments to encourage patients to participate in screening programmes. This advice has often been criticised for not being honest about the harms of screening (the controversy in the NHS about patient advice is summarised in the BMJ here). The article argues that even the revised NHS leaflet fails in its duty because:
No mention is made of the major harm of screening—that is, unnecessary treatment of harmless lesions that would not have been identified without screening. This harm is well known and acknowledged, even among screening enthusiasts. It is in violation of guidelines and laws for informed consent not to mention this common harm, especially when screening is aimed at healthy people.
A previous analysis of advice to patients from other health systems argued:
The major harm of screening, which is overdiagnosis and subsequent overtreatment of healthy women, was not mentioned in any of 31 invitations.
The early days of screening also happened to coincide with significant improvement in breast cancer mortality which was ascribed to the benefits of screening. In principle, however, mortality can be improved by better treatments which would have been implemented even in the absence of screening. Recent evidence has tipped the balance away from screening (see this BMJ review comparing different European countries).
So there are several reasons why many people will overstate the net benefits of screening. as better evidence emerges it is likely that that evidence will provoke controversy.
Update February 2014
Another large long term and properly randomised study (in the BMJ) has concluded that screening doesn't even reduce mortality from breast cancer (at least when good therapies are available). In their words:
Annual mammography in women aged 40-59 does not reduce mortality from breast cancer beyond that of physical examination or usual care when adjuvant therapy for breast cancer is freely available. Overall, 22% (106/484) of screen detected invasive breast cancers were over-diagnosed, representing one over-diagnosed breast cancer for every 424 women who received mammography screening in the trial.
So screening does no good but comes at the cost of significant over treatment (women suffer surgery and other clinical interventions that do harm but create no benefit).