Persons at least 65 yrs of age should undertake an informed cost-benefit analysis before using OTC anticholinergics, including first-generation antihistamines, such as Benadryl or Unisom. This age group was the subject of an excellent, long-term study with the results reported in 2015:
https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2091745#:~:text=Results%20The%20most%20common%20anticholinergic,79.9%25%5D%20developed%20Alzheimer%20disease).
A 2022 publication of the Harvard medical school includes a discussion of the highlights of this 2015 study:
https://www.health.harvard.edu/blog/common-anticholinergic-drugs-like-benadryl-linked-increased-dementia-risk-201501287667 B/L? Long-term use (at least 3 yrs) of an anticholinergic raises the risk of dementia by 54% compared to short-term use (no more than 3 mos) over the 7+ yrs of average participation in the study. One of the three main types of anticholinergics studied was first-generation antihistamines. The Harvard article explains that we oldsters don't produce as much acetylcholine as we did when we were younger. (Acetylcholine is an important neurotransmitter, and our production of acetylcholine depends on cholinergics.)

The Harvard article mentions an anticholinergic cognitive burden scale prepared in 2008 by an Indiana University medical school geriatrician, which includes among its recommendations the use of second- over first-generation antihistamines.

So, there's the downside to the use by older persons of presumably harmless old-school antihistamines--a risk, but not a certainty, of (further?) cognitive decline. On the other hand, each older patient must decide for himself or herself whether the upside is worth the risk. The decision is worth some thought beyond denying the risk because first-generation antihistamines are OTC or because one knows other older persons who take the same antihistamine and haven't suffered any cognitive decline.