Looking around your room and feeling like it belongs to a younger kid means the person living there has moved on.

Outgrowing a space is one of the more reliable signs of internal change, because you did not decide to feel that way about the posters and the shelf. The room stayed exactly as it was. Your relationship to it moved, which means something in you already did.

Rather than reading that feeling as sadness, treat it as information. Change a few things in the space to match where you actually are now. Keep what still means something. Let the rest go without a ceremony.