Timing for changing filters for RO unit

I'd like some input on changing the carbon & sediment filters on my RO/DI unit.  I know all the standard literature about changing them every 6 or maybe 9 months.  And that you want to keep the TDS low coming out of the RO unit so as to put less work on the DI resin (as that is more expensive) than the other two filters.   So.......I've been running my filters now for probably 15 months.  But I'm having a hard time justifying changing them for new filters.  Here's why:
My current TDS readings are 10 (after the RO compenent), and 0 (after DI).   While that may seem a reson to replace the filters, the problem is that when I replaced the filters 15 months ago, the best reading I had after the RO compenent was only 6.   I determined that that was a reasonable expectation for the filters, since the water in West Des Moines is notoriously bad - around
250-300 TDS.   So, during the 15 months I've used the current filters, the TDS has only gone from 6 - 10.   So are there other reasons I'm not aware of for replacing the filters. 
My unit is rated for 45 gal/day unit.  I use it probably once a week - as I only need 10-15 gal/week for my 75 gallon tank.  Each time I run the unit, I open the flush valve and flush it for maybe 30 seconds - 1 minute.  I watch the TDS meter when I initially start up the unit.  It starts at about 150 TDS, but quickly makes its way down to 9-10 TDS - over the course of 1 or 2 minutes.  I also check TDS when I finish and its always at 10 or maybe 11.  I've had the unit for 4 years - so the membrane is that old also - I've not had to change it yet either.
Any thoughts?????