Whether you choose a Traditional martial art or a sport should be determined by your goals. If you have dreams of going into the UFC and becoming the next champion of the world train in a sport. If you determine that you want to learn a life protection art that will enable you to protect yourself or your family choose a Traditional Art.
You age has little to do with other than if you are 40 it is not likely that you will get a title shot. You might get some matches and make a little cash, but you probably won't earn a good living being new to the sport.
Being in your early 40s is not to late and I have seen adult males start in their late 50s and 60s even. I do think you want to take a realistic approach to this though and understand that your body will not recover as quickly from hard or grueling workouts as when you were younger. Also any training injuries will take longer to heal probably so avoid them as much as you can.
That means taking time to warm up thoroughly before stretching and working out and doing some cool down stretching afterwords as well as listening to your body and getting enough rest and hydrating properly and all those other good workout habits that sometimes younger students ignore. Also look for a class that has some older students in it and an older, more experienced instructor preferably who will be understanding of you not being a teenager and not able to do some things like some of the other younger students can do. Flexibility is one thing that comes immediately to mind and it will take you a little longer to build that than someone younger.
As for competing they have divisions for older, beginner students at larger tournaments but the contact can be a little hard from what I have observed in those divisions. This is due more to a lack of skill and control than anything else usually though. Competition and sport karate is something slightly different than traditional martial arts and what the focus is on in general in class and for testing and promotion.
I mention this because you might just want to focus on the traditional aspects to start with for the first year and starting to build a good foundation in that before delving into competing. Enjoy and good luck.
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