At age 70, they are still working.Without the mindset and knowledge of what it means to invest wisely and have passive streams of income, hard work is wasted.   

It does however become more difficult.Yes you made the age old major mistake in the beginning as every new landlord does of fall into the trap of trying to please their tenants and be the "good' landlord.

I mean they both have the same or similar amount of bedrooms, bathrooms, age, sq ft, under a mile radius, etc.

We're about the same age, and feel the same way.  

What I was good at was running a business, I started my own at the very young age of 19, and what I did have was the desire and determination to succeed at my new goal at that time - real estate.Start small and flip a house.

Are the dependents over the age of 18?

- In terms of market due diligence, what is an optimal target population, what percentage of such demographic should be 50+(or if that needs to be of higher age), and what about net worth/income?

In my local area, most properties are being bid up well over asking price so if you try to offer at or below asking, the selling agent will ignore your offer.A final note, you can also filter deals by the age (Days on Market - all the way to the right side) and find stuff that has been sitting there.  

Of course roofing and year of ac unit matters, but do investors really focus on the age of the house?

The lot came with a mobile home not considered to have any value do to its age (40 yrs).