I turned 40 in 2000. I had finally finished my education, I had a full time job, a marriage, a house and a mortgage. I had the money, if not the time, to stash yarn to my heart's content. My husband blithely told me that "We won't go hungry with you buying yarn," so I had permission to buy as well. (I chose not to tell him about the then $40 and $50 skeins I could buy, since I didn't plan to spend that kind of money on yarn anyway.)
Then I joined a not so active knitting guild and a very active crochet guild. Several times a year, I would travel with crochet guild to various events, and we always found a yarn store to visit before we headed home. There was always yarn to adopt at the meetings, too. Needless to say, my stash grew by leaps and bounds.
Then the 2010s hit. Our elderly parents suddenly needed a lot more care, which ate into the limited time I had to craft. We lost my father-in-law in 2013, and my mom in 2016. After my mom passed, though, I found that I had quite a bit more time on my hands, and I began to get reacquainted with my stash. It was an eye-opening experience. It turns out that yarn doesn't date, but color does. I realized that I was going to need a plan, and a good eye for color, if I was finally going to put the stash to good use.
Happily, I had been playing around with color for a good few years. In 2009, I started an afghan called "the never-ending blanket", which is basically a solid granny square that you can make any size you like. I used yarn scraps and a color wheel to get a sense of pleasing color combinations. It was good practice that I now apply to my larger stash as well. Recent yarn acquisitions in more current colors have also helped. The challenge of using up the stash remains, but it is a very happy challenge indeed.