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.
I use Krita for everything, I love it so much. I also won’t act like it’s perfect either, despite it being my most used software by a landslide. Personally my biggest desire now is improved workflow for text editing (e.g. editing text directly on the canvas, being able to box and justify text, vector pathing for text so you can make it bend or wave). From what I understand it is something that is being worked on, and I will be even more indebted to the wonderful folks at KDE once further progress is made on that front.