I've spent way too much time digging into character backgrounds. Nessa's age isn't explicitly stated in 'Pokémon Sword and Shield', but based on her design and role as a Gym Leader, most fans estimate she's around 16-19 years old. She's clearly younger than seasoned trainers like Leon but older than rookie challengers. The Pokémon universe often keeps ages ambiguous, but Nessa's confidence and position suggest late teens.
Her youthful energy and modern fashion sense align with older teen characters in other Pokémon media, while her mastery of Water-types hints at several years of training. Comparing her to other Gym Leaders like Milo (who seems slightly younger) and Raihan (who appears older) helps place her in that sweet spot. The developers likely wanted her to appeal to both younger players and older fans, which explains the ambiguous but relatable age range.