Growing old with someone by your side does not sound like a realistic life goal for a BL unless you are also able to fall in love with adult men or women.

I don't know of any BLs who have grown old with their YF. I know BLs who continue to have a friendship with their YF into adulthood, but in every case I know of the YF eventually goes on to marry someone else and make their own family.

Most likely, you will have to come up with a different life goal unless you are willing to explore adult relationships. There are many happy lifestyles available to us. But living with our YF for ever is unlikely to be one of them.