indoor cat trys to beat outdoor cat sometimes?

I have a old 15 year old indoor cat that sometimes acts strange he trys to beat one of my outdoor cats once in a while when she comes inside from being outside for a while

it doesnt bother the outdoor cat cause shes a lot faster then him and she doesnt seem to take much notice of it just hisses and will take off but its strange he never did this before? He use to get picked on by her but now seems to be turning the tables

Like people, when getting old, odd behavior happens. I also have a senior cat but she's female. She used to prefer being left alone but now she sometimes goes to the young ones and gives them a kiss. Then she goes back to her snobbish self.
The young cats seem to accept her behavior. I think your outdoor kitty does too. The hissing may just be her way of being surprised at the "old man's senility"?

Yep crankiness in older cats is kind of funny

My oldest alpha female can be a sweetheart one minute and a complete the other.

It could also be smells of the outside. If your outside cat has been around other cats your older indoor cat my smell that and be reacting to that rather than the younger cat.

I would not be to concerned. Cats are cats and it sounds like a pretty normal thing. Heck my three indoor cats can do that out of nowhere for no reason other than maybe they are having a bad day