I'm not sure, but you could always make a metaphor. Might I suggest the word sloughing, which is generally used to mean the shedding of dead tissue? Like say, "no matter how hot you work the iron to slough its form-". Though then again, might be a big ask for a transphobe to use a rhetorical device like that.
I posted this at at 6:30AM on GMT-5, and the farthest out time zone due to some funny quirks is GMT+14 for the line islands. So to everyone in like, New Zeland and Christmas Islands, I sincerely apologize!
It feels like a toss up to me as well, but I'm going to try my damnedest. I crossed the 24hr mark because I wasn't exactly planning to do this, but now I feel weirdly invested. We got the same clock, 6 hours to go!
To our fallen soldiers, know that Luigi appreciates your resistance against adversity. Luigi respects your contributions to incremental change. Luigi knows perfect is the enemy of good, and just how much we need good right now. Do not feel shame in failure, but solace in knowing you tried in earnest.
If you just mean your relatives, it's important to know that what constitutes 'young' changes as you age. As someone in their 30s now, early 20s and below feels young to me personally. If your relatives are much older than you, especially if they saw you growing up, it's likely you'll seem young to them forever. That isn't to say you should be infantilized your whole life, but just that it stems from differences in perception. If you feel like you're being treated lesser than other adults, try to tactfully explain how you feel. Maybe you're feeling excluded and that hurts, or perhaps being called certain things is belittling to you. Ultimately though, the most important thing is that tact, because it's very likely not malicious, your relatives likely aren't trying to make you feel small or invalidated.
As for anyone else in the street, or perhaps online, there's usually something that makes people naturally assume youth. I don't know what you look like obviously, but online you typically have very little information to work with. Usually just the words that are spoken, and maybe an avatar to go along with it. A number of people have pointed out your typing style already. I won't belabor the point, but I will say first impressions are extremely quick to make, and quite influential. I personally try to talk to anyone with tact and just 'as a person', but it's something I feel doesn't come naturally. I really only ended up doing it because I spent a long time in moderation type roles, having to mediate issues with people and events.
Tl;Dr: It sucks, but people probably aren't doing it to hurt you. Talk patiently to them, also type more formally.
We're rats, we're rats, we're the rats!