This is all in your head.

Enjoy them as individuals. That seems to be how they are seeing you.

I have both younger and older friends. That is not odd for adults.

Im 51 and one of my closest friends just turned 80. Friendship can transcend age.

We had a friendship with a couple. They were 20+ years older than us. We were right in-between the age of their adult children and themselves. I think we filled-in for their adult children, sometimes. They use to call us when they wanted to get together but the adult children had their own plans. We had some great times together.

I became friends with someone in the neighborhood 20 years older than I. It had a little bit of a feeling of an older sister (I don't have any sisters). After awhile, almost a decade, I did wish it to be more like equals. I didn't quite feel that I was being respected as her equal. So, that has hindered our relationship a bit in the long term.

They are probably having plenty of get togethers without you. So, there's that.

Enjoy whatever the four of you can share!

OP, don't make it a thing