A median year housing unit was built puzzle

I recreated your table call and looked in the "Notes" section (when you click "Customize Table", notes is in the upper left hand corner in the bar across the top). I found two notes that refer to the negative/dash symbol in the estimate cells:

• An ''-'' entry in the estimate column indicates that either no sample observations or too few sample observations were available to compute an estimate, or a ratio of medians cannot be calculated because one or both of the median estimates falls in the lowest interval or upper interval of an open-ended distribution.
• An ''-'' following a median estimate means the median falls in the lowest interval of an open-ended distribution.

Since this is a median estimate, it looks like the median year is very close to the lowest year in the distribution ("falls in the lowest interval"). When I looked at the estimates for the 5-year ACS for 2013 and 2015 (on either side of this), the years are very similar. In the Hawaii census tract, the median year in all three cells hovers around 1974. In the Texas census tract, the median year hovers around 1990.

To get around this, I would use a different year, if possible. If that is not possible, you could average the estimates in 2013 and 2015. Both 2013 and 2015 share 4/5 of the same data as 2014 so averaging them will give you something close to what 2014 would be.