I am female and 73 years of age, coming up 74 and do not have a partner of any kind (having been on my own for 35 years. I have owned and managed my boarding kennels for just over 20 years, and I try to do puzzles and the GK crossword at least every week. I also manage my bank statements, keep track of the cash flow in the business, and am currently negotiating new contracts for 1st April with a veterinary surgeon and three local councils without employing a lawyer.
I agree entirely with the "try to keep your mind stretched" responses, but if worried see your GP, providing you have a good relationship with that individual doctor.
I forget names temporarily, (eg my junior staff) but nothing long term, still know what my car keys are for (although frequently putting them down and forgetting where I put them),
I find getting up early, working most of the day, cooking proper meals (I have my grown up son sharing my house, and I have an Aga) , watching for relaxation and going to bed early appears to keep me on the right track.
Good luck to you, Ann