I dug through every official source I could find and, honestly, there's no tidy little age tag for Nessa tucked away in the game files or the standard guides. In 'Pokémon Sword' and 'Pokémon Shield' the devs give gym leaders personality, backstory hints, and visual design, but they don't embed a clear age field you can read off like a trainer ID. What you will find in-game are dialogue snippets, gym leader bios on promotional pages, and artbook-style
blurbs that describe who they are — but not an explicit birthdate or numeric age.
If you want to search like a curious player, check localization text, NPC metadata and any promotional profiles on The Pokémon Company website or press kits. Fan communities and wikis often collect every canonical line and official blurb and then offer estimated ages based on design cues, voice actor ages, or interviews. Those estimates can
be useful for headcanon but they're not official. Personally, I enjoy how the mystery lets people project different takes onto Nessa — some see her as a young
prodigy, others as a composed early-twenties athlete. It's part of the fun for me.