How old is too old? (or, what should I know about homes built in 1920?)

[Taylor Kendrick]

Hi everyone,

I'm looking at a variety of multi-family homes right now and I found an interesting MFH. All brick, built in 1920. Much older than most of the other listings I'm into but the rough numbers indicate it would be pretty profitable (+$2,000/mo). Conservatively maybe I get more like $750 - $1,000 net profit per month.

I actually quite like the property overall. It reminds me of my grandpa's home which was built in 1924. It's been sturdy as hell but certain things (like plumbing) were a pain to modernize for him. 

What does the squad say on this? How old is too old? What do I need to look out for with a property from this era? Do you draw any hard lines in the "Year Constructed" sand?