I presumed it was cortisol/stress, which can cause aging.

By contrast, the study found that despite the stigmatisation associated with social renting, it was not found to differ from outright ownership in terms of association with biological ageing. Social renting is typically lower cost and offers greater security of tenure than private renting. The authors stressed that the DNA methylation-derived measures are relatively new and more data will be collected to assess how biological ageing markers change over time.
Private renters typically live in worse conditions that other householders, with greater problems such as cold, damp and disrepair. The English Housing Survey estimated that in 2021, 23% of private rented homes did not meet the Decent Home Standard, compared with 13% of owner-occupied and 10% of social-rented homes.

However after reading this, it might be more due to living in disrepair, since the effect goes away when living in public housing, (which must be much nicer in the UK than in the US based on this.)

And community, hard to know your neighbors when you’re renting.