。
The elderly tend to dwell on the past.
Literal
Old-people [topic-は] past [object-を] look-back-tending [copula-である].
~がち ('tend to, prone to') is a suffix indicating a habitual tendency, often with a slightly negative nuance. 振り返りがち = 'tending to look back.' ~がちである is more formal than ~がちだ, fitting the analytical/written register. The construction frames the observation as a soft generalization rather than a complaint.