Does liquid 5% Minoxidil stop working after a certain amount of time?

The most likely cause is more and more genes are being expressed inside your scalp and hair follicles that are driving the balding process forward. There are over 300 genetic regions involved in balding. At 16 you might have had 4-6 genes expressed at the start of your balding. At 20, there might have been 10. Now there could be dozens and dozens of genes expressed. Minoxidil is usually working the same. While it was pretty good at stopping 4 genes, it can't hold back the genetic changes associated with 60 or 70 genes. These numbers are different for everyone and for you specifically they may of course be different- but it illustrates an important point. The scalp environment and hair follicle milieu changes drastically over time.

It is well known that for many people with androgenetic alopecia Minoxidil can stop being effective. Your options are three-fold: LLLT which works well for 35-40% of patients; finasteride (not recommended for young males due to sexual side effects) and PRP. You can do both LLLT and PRP at the same time which is what I would suggest. 

The half life of minoxidil is about 22 hours.  You should try to apply it twice daily as long as you feel your are losing hair.  If you stop doing it the effects generally wear off rather quickly.  Minoxidil does nothing about DHT however.  For treating that, you need to be on finasteride.  I would recommend you seeing a hair loss specialist right away and getting a master plan in place for next decade of your life to be sure you keep as much hair as possible.  PRP or other cellular therapies a generally a good option as well... Look into them!!

There are a number of reasons your hair loss may progress while using minoxidil.  If you have changed generic minoxidil brands in the past 6-12 weeks, there could be an efficacy issue, or you may add a second daily application .  If you are not using finasteride, the effect of DHT on natural hair loss progression may now becoming evident.  It sounds like you have significantly slowed down hair loss progression but it may be time to add other treatments including hair restoration surgery

There is no guarantee that minoxidil will continue to work. In some people it works for a long time, for other it does not