From a methodological point of view, the study is correct. It uses a good cohort and uses appropriate techniques to calculate methylation age. Methylation age can reflect biological age, and the deviation between actual chronological age - the age marked by your date of birth - and biological age is linked to lifestyle, whether you take care of yourself or not. With many nuances, biological age is also associated with health status. 

Steve Horvath is an American mathematician who, using DNA methylation data, discovered a number of sites in DNA whose methylation status correlates very well with an individual's chronological age. Horvath's work had a huge impact on our field [about 10 years ago]; until then, telomere shortening was used [to measure biological age]. 

In this study, the team uses people's DNA methylation data [from a UK database] to calculate their methylation clocks. Looking for associations between different variables, they conclude that living in rented accommodation is associated with higher biological age than living in a home of one's own. Although I do not know enough about the situation in the UK and I am not a specialist in social sciences, I do not think it is realistic to extrapolate the results to Spain because each country has different socio-economic circumstances. 

I think this study is an interesting example of how our genes interact with the environment and chance to determine who we are at each moment of our lives. However, it should be noted that this is an observational study and therefore does not demonstrate a cause-effect relationship. Moreover, although the authors have taken into account other variables (such as socio-economic status), we cannot rule out that there are others involved that have not been taken into account.

This study adds to existing knowledge about quality of life and how factors related to physical and mental health accelerate or delay biological ageing (or epigenetic clock). The study was carried out in a population of 1,420 people in the UK, who were asked questions about their past and present housing and their feelings about it (whether they would like to move but cannot, etc.) and blood samples were also taken to analyse biological ageing. 

Biological ageing (or epigenetic clock) is something like a clock that indicates what the age of your cells and body would be and is a different concept from chronological ageing (a person's age). That is, a person can be 80 years old but have cells that have traits and behave like those of someone younger. In fact, there are studies that indicate that people who reach very advanced ages have a very slow biological ageing, that is, at a biological level, their cells are like those of younger people (and this is probably why they have reached such advanced ages). 

Moreover, as this study indicates, biological ageing or the epigenetic clock can be reversed, i.e. we can 'rejuvenate' at the biological level, mainly through changes in our lifestyle and quality of life (less psychological stress, good diet and physical exercise, and in general everything that involves physical and mental health). As the authors point out, the possibility of reversing biological ageing provides a scientific basis for investment in policies related to access to quality housing, as improved quality of life delays biological ageing. 

It is important to note that biological ageing (or epigenetic clock) can be measured in different ways, i.e. there are several characteristics of our cells that can be measured. It is worth noting that the measurement used in this study is largely supported by previous studies. 

In fact, the conclusions of this study are unsurprising, in the sense that they are consistent and expected given previous studies. This does not detract from the merit of the study, for while it would be expected that living or having lived in poor housing conditions, independent of other factors, would delay biological ageing (or epigenetic clock), all hypotheses (theories) need to be confirmed by studies. 

Perhaps the most relevant aspect of the study, in my opinion, is that having a mortgage does not affect biological ageing either positively or negatively, which is surprising, as a mortgage is often a source of psychological stress. In fact, the study does not comment much on this fact, but focuses on comparing renting and home ownership, but I would like to ask the following question: is it possible in Spain to have access to (quality) housing without first having a mortgage and without having problems paying it due to job instability? In any case, as I have already indicated, the study does not conclude that a mortgage accelerates biological ageing, although (and this is relevant) having had problems paying it does accelerate it. Hence the study proposes more facilities and aid for accessing quality housing. 

In any case, the study was carried out in the United Kingdom, a European country with a welfare state more or less similar to that of other European states, and seems to be extrapolated to Spain, as the factors considered as accelerators of biological ageing are basic necessities: too many people in a very small house, defective roofs (this may be especially relevant in the UK because of the rain, but in any case it denotes a low quality of housing), inadequate heating system (this is perhaps not as important a problem in most of Spain as it is in the UK).  

The authors point out that paying rent is a factor that has a much more negative impact than the factors I have just mentioned, and even appears to be more negative than unemployment (when compared to having a job) or having been a smoker (when compared to never having smoked), although these latter conclusions should be taken with caution. 

In any case, it is an interesting study, which, as I have already mentioned, is in line with previous studies, and supports the implementation of social policies not only for ethical reasons (the right to housing), but also because delaying biological ageing and, consequently, the diseases associated with biological ageing, improves people's quality of life and, therefore, represents an investment in preventive medicine.

The study examines whether housing circumstances are related to faster ageing as measured by biological markers. Previous studies have already linked adverse housing conditions to poorer health. This study provides insights into the biological mechanisms that may be behind this. They find that private rental living, continued late payments or exposure to pollution are associated with faster biological ageing, which is estimated to be about half the equivalent of smoking or double that estimated for obesity. This fits with the known literature where we know that financial stress, residential insecurity or exposure to pollutants within the home can lead to poorer health. 

The study has a good design with use of UK biobank data. However, there may be some biases in the study, especially because of a very selected study population, with questions still to be answered about how it affects people with other circumstances of racism or whether it also leads to premature ageing in younger people such as children. The study may have relevance as the biological ageing process is potentially reversible, so better housing conditions could reverse these effects.

This paper raises the possibility that living as a renter may have a negative influence on health and on the speed of ageing. To investigate this possibility, DNA methylation is studied in a large sample of the population.  
It is known that DNA methylation acts on the functionality of certain genes, usually by inhibiting their expression. Younger organisms have a greater number of methylation sites than older organisms. The rate at which this methylation decreases with age depends on many circumstances and marks, in a way, the rate of ageing. 
Although the study is statistically consistent, the conclusions cannot be considered definitive given the interaction of multiple factors in people who cannot afford their own home. Isolating just one of these circumstances may bias the study's conclusions.  
However, the paper delves deeper into the impact that different stressors and, in this case, living in rented accommodation, have on health and ageing. These circumstances should be taken into account when planning social intervention policies.