I'm being hyperbolic to demonstrate that the numbers may be what you want to focus on instead of age.Took a look at this post too and as far as I'm concerned it's all about risk tolerance and your exit strategy (which is different from investor to investor). 

To the extent anyone is able to, and based on personal risk tolerance and investment objectives, you would do a little bit of everything and diversify across many different asset classes (real estate, equities, fixed income,commodities, and other alts).    

@Slocomb Reed, you can search numerous threads or Dodd Frank regulation, but what you will find can be summarized as followsDodd Frank Act, and administrative rulings by the Consumer Financial Protection Agency, are unbelievably numerous, hard to interpret, subject to subjective judgements, have harsh and draculon penalties for even small or inadvertent infractions, and cases are being pursued by trail attorneys both as individual cases and as part off class action.So, a lender set up to do a huge volume of personal mortgage loans may find the risk acceptable, the enormous investment in compliance and personnel  and procedures a cost of doing business.Private lenders do not have the time, money, staff or risk tolerance to be in this market. 

It's all about your risk tolerance at that point.

Whether you pay off properties or not I think is a function of your risk tolerance and what phase of investing you are in. 

If you can pull off more given the DTI, credit, etc. qualification limits and your personal tolerance for constantly moving and living with your tenants, try to repeat the process one or two more times.  

All of it is really off and only tolerable with workarounds.• There is no real way to make super basic and functional reports, that are typical of multi-family.

@Morgan MadillThis is a question of your own risk tolerance.

Meaning they are less tolerant of bad borrowers.

However, this rate can change periodically, leading to potentially more significant monthly payments in the future.When deciding between a fixed-rate and adjustable-rate mortgage, it's essential to consider your financial goals and risk tolerance.