I can't altogether dismiss your concerns. I saw my mother take care of my twenty-years-older father as his health declined and Alzheimer's took over...not pretty.
But...you never know what's going to happen. A hot, healthy, 25-year-old partner could still die, or become disabled and dependent. Chances are less with a younger partner, but there's no guarantee you won't find yourself a twenty-something widower, or a caregiver, even if you end up with a partner who sees you as the "older guy." Or YOU could be the one who ends up disabled, and your older partner could be taking care of you! Stuff happens, and it can come at you when and where you least expect it. You can't live a life without risks.
This guy is 55--not exactly one-foot-in-the-grave just yet (even if 55 does seem ancient from the perspective of 25). Could you settle for a couple of good decades with a great guy, maybe more, assuming it all works out? That's more than a lot of people ever get. If that's an attractive enough prospect to get you past your fears, then hang in for a while and see where it goes.