If we are in a group, it is quite natural to feel neglected or left out at times, but as I can read in your case, you feel you are being left out simply because you are elder to them.
First of all, it is not good to assume things and facts on your own. Who knows they simply ignore you for this reason? It is better to communicate because you might think that it is important for you to sound funny, but these group conversations are really confusing.
You can start by giving them advises on their night out. Now, the reason why I am saying so is that even if they consider you elder to them, so much so that they feel you of no use in their group. Being mature, you can be of some good help to them. For example, for a road trip at night, you can ask them to keep hockey sticks or pepper sprays with them. If you have already learned some defense methods, then it will be great for them too.
Know the Reality of Group Fun - You know plans are meant to be spoilt. How many times you have made a plan for an outing to Goa, and it has become a mess. Now, at that time, was it your mistake? Certainly not.
Check Your Body Language - You must know whether your body language is not irritating. Because, if that is irritating, then you will not only feel neglected in the circle of friends of yours but in any group communication.
Do Not Be Dominating - If you are elder, that does not mean you will strictly give orders, or always want to initiate. You should politely raise your voice and be a good listener too when needed.
After applying the aforementioned advice, I hope none can feel cornered. Have a happy night out!!