As someone who has trained since I was a child, through my teenage years and who is now embracing the 40 milestone I have first hand experience with how to adapt your training as we get older.
Here is some advice so you can practise martial arts for another 30 years :)
1: Listen To Your Body. If you started martial arts when you were young or used to practise when you were young you will want your body to be doing them jumping flying kicks until the end of time. However, what you could once do easily, will probably now be that bit slower and not as effortless. Remember that's fine. Your body is not letting you down in any way it's simply telling you that if you want to continue enjoying your training you need to slightly adapt. Listen to your body and embrace your age. Remember getting older means physical aspects decline but wisdom and understanding increases.
2: "Long term consistency trumps short term intensity." It was Bruce Lee who was acknowledged for this quote and I think it's so true. What would you rather do? That one training session where you give 100% physical effort and injuries are weekly occurrences and your body is constantly in recovery and pain OR training three to four times a week, working at 70-80% and learning more and improving. It's really a no brainer. Push that self preservation button. It means you can actually enjoy your training and still keep fit, strong and healthy but not constantly be injured.
3: Open the door to new areas of learning. When we are young our enjoyment in martial arts comes through being physical. A good workout means a great session. Some people associate a lack of sweat when training to a lack of improvement and that couldn't be further from the truth. Martial arts is a journey of self discovery. Most of this is done whilst meditating or practising energy movement and philosophical exercises. Most of these avenues we don't pursue when we are young however, as we get older we learn that these are the most important aspects of martial arts. So read books, study more, understand and study energy movement, history, yourself, your mind and others. As we age our mind develops. This is why it's impossible to have a child Master grade. A Master is someone who has a great understanding of their body and mind. Children tend to only have a grasp of one of them concepts.
4: Keep those joints stretched. Get those vital stretches into your daily routine. Stretch that back and keep it mobile, work that core and stretch your joints regularly. Body maintenance when we get older is vital if we want to keep going as long as we can.
5: Concentrate on breathing exercises. Breathing is the foundation of any martial art. If we can master our breathe we can massively increase all physical aspects but more importantly we can tap into energy flow and controlling our mental health. Study breathing and let it open up a whole new realm to your martial art training.
6. Practise for health. Let your emphasis be on maintaining and improving your health. It's as simple as that.