Heads up for adults using youth day passes
From my experience riding caltrain for years, there's a less than 50% chance of an inspector asking to check your ticket on board. Most of the time they don't. And when they do, they usually take look for 2 seconds to make sure you have a ticket and then go on their way. So it's pretty easy for people who are over 18 to get away with paying $2 for a day pass and claiming to be youth.
Today, a group of boys (who I assume were in college) got unlucky on a train southbound from SF. They all had youth passes, and an inspector passed by and asked them all for their date of birth and asked to see ID. I didn't hear all of what happened, but one of them said 2006, and the inspector then wrote all of them $75 fines for being adults using youth passes and lectured them for 5 minutes. I believe one of them was actually 17, but I saw the inspector holding at least one boys ID. He had initially told them to get off at the next stop, but they ultimately stayed on the train.
The inspector had also asked me my date of birth, and he said that starting next week they were requiring IDs. He also told this to the boys he caught and someone else nearby who had a youth pass. I've never had to show my ID before on a train so idk how true this is, and maybe this is just one super strict inspector on train #124 on a random Thursday evening. But just as a heads-up to those who are using youth passes and aren't actually youth, you might not get away with it forever.