Start now, because that insecurity only gets worse with time. The beginner class is exactly for people with no experience, regardless of age - the instructor is expecting people in their 28s, 38s, 48s. What you see in videos is either people who've been training since they were kids, or folks who already have some years of practice under their belt. Nobody's born knowing this stuff.
The point where most people mess up - and what the others didn't mention - is getting too worried about keeping up at the beginning. You're going to feel slow those first few months, it's normal, but it passes quickly because your body learns fast when you train consistently. The real difference isn't between starting at 8 or at 28, it's between someone who trains twice a week for a year and someone who goes once a month when they remember.
Focus on picking a place with a good atmosphere - somewhere the instructor actually cares about the beginner class and doesn't ignore it in favor of the more advanced students. Then just start and stop overthinking it.