I found myself starting from scratch with physical activity as an adult, and what surprised me most is how quickly your body adapts when you do it consistently. It's not a matter of age - 34 is a perfectly normal age to start karate. What really works is finding a gym that has specific classes for adult beginners, so you don't end up thrown in with teenagers doing advanced techniques.
One practical tip that almost nobody mentions: when you go to check out the gym, explicitly ask to watch an adult class, not just talk to the instructor. See how people move, how the sensei explains things, whether the atmosphere is truly relaxed or if there's pressure. Then if you feel the right vibe, start at your own pace - the first few months are tough from a breathing and muscle perspective, not technique, and that's completely normal. Karate is also a sport where you don't directly compete with others at the beginning, so even if you're slow to learn a form, nobody "beats" you while you're starting out.
It's not better to choose something else, really. Fear of making a bad impression is the main enemy, not age. Everyone in that gym went through a terrible first month, including the young guys.