This is a very normal way to feel. You had children at a time when others were already significantly along in the process. You made a choice that was based on your needs and was not simply an arbitrary matter. You did what was best for you at that time. And, it is wonderful that you have twins. That is a terrific and marvelous thing, one that most people do not experience. But, there are good and bad to all choices, and in the short term this can be very stressful and isolating as you have been pressed into a life stage that the others have already completed.
The best news is that this isolating stage will not last. As your children develop and grow out of their high needs stage (about age 1-1.5) they will become a big and essential part of this social group. True, they will not relate the same to this first group of children as there is a large gap in age. But they will have the support and modeling of a group of teens-preteens just when they need it. And, that is developmentally wonderful for both of these groups. Both will need each other in he near future. Simply said, it will not remain so isolating and lonely all the time.
But, for a brief period of time it will be harder as you have a huge development t gap. You have babies vs their "kids". But the speed of your children's development will equalize this fairly quickly.
You are also at a point in your own life where you naturally feel discouraged. 3-5 months after the birth of children is the (no kidding) lowest point on the life satisfaction scale: The stress, fatigue and exhaustion just play havoc with your emotions. It is very easy at this point to feel like you have done nothing correctly and that you somehow missed the bus so to speak. You did not. It was a good choice for you and it will equalize. It is that this is, right now, lousy.
Feeling left out is no fun. It makes people feel everything from unwanted to unimportant to rejected and even betrayed. But, again, this will correct itself in due course. You did nothing wrong in waiting to have children. It was a good choice and it was right for you and... look at the wonderful results. Long term this will fade quickly, but for a while it is normal to feel so odd and isolated. Steven