I had my THR at the ripe, old age of 42. Like you, I got the "you're too young" line a lot. My THR was the result of a hip fracture, so I got to hear that I was too young for that too.
I definitely got tired of hearing it and feeling like I needed to explain things to people. I came up with two stock responses. One was to roll my eyes and say, "yeah, I get that a lot." The other was to look shocked and gasp, "I've never heard that before!" I think both of those may make people think, because I think that, very often, when people say inappropriate things, it's coming from a place of thoughtlessness than deliberate rudeness.
One thing I can tell you is that at 5 years post-op my hip is not an issue. Only my family and closest friends know that I have an artificial hip. Once you've recovered, it doesn't have to be a topic of conversation unless you want it to be and people don't have to know about it unless you want them to.
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