I am indeed funny. Forgive me however. I might have written a snarky answer here, but I'll try answering your query in earnest.

I have a developmental disability. This means that I developed and still develop in an uneven way. There are specifically diagnosed delays. While I don't look particularly young, I act in very childish ways because the genes that regulate pruning as well as other processes are not functional, or limited in their functionality. I have not developed much since turning fourteen. This milestone happens to be around a decade old. This person you're seeing answering your question is more or less embarrassingly a child and they have difficulties taking care of themself.

This is also obvious from my behaviour. I intone like a kid, I nag like a kid and I am interested in age-inappropriate topics. When I was nine to ten I would watch programming for toddlers. I'd genuinely enjoy it. When I was nineteen I became interested in anime meant for middle schoolers and so on. To this day I have really odd expressions especially when I am happy or excited, those that you would expect from a hyped up fortnite kid.

People just happen to notice that. You may not look younger but you act like a child with autism and that makes people think you are younger.