Well I did some research and looks like the owner is around retirement age.

They're all working until at least age 60 and paying lots in federal taxes.

You and I both could build ground up, but at my age I sure wouldn't want to, much less deal with the city.

My partners and I have a property under agreement, however we are both under the age of 55. 

As @Zach Lemaster mentions above, the key is to manage your rental portfolio and exchange those where the major systems are starting to age before you get hit with the bill and then have to hold the property for several more years to recoup the cost.

But I began this reply with IF, because if they are all cash-flow positive, I would personally not seek to pay any of them off early (depending on your age), and instead save that money to put towards obtaining additional properties.

Not only were the phone numbers, addresses, and email addresses better, I was getting a much more complete data set showing me DECEASED indicators and ages as well as the names and associates of relatives I could contact.    

I'm teaching folks from ages 15 to 60+, some of whom have no net worth or a negative net worth.

This could become the basis for taxation.There are multiple methodologies that are utilized by the tax assessor when determining a property's assessed value and which one is utilized is dependent on age, property type, region and other factors.

The original insurances are covering the original purchase price of the properties, which are 25% of the worth in this day and age.