Does it happen that natural blond(e)s stay recognizably blond into their 30s and 40s? Will exposure to sun make a difference?
All the blondes I knew in school or at uni had turned mousy or peroxide by the time they were 35, so I have no data from observation.
(Background: I have a character who was pale blonde as a teen, and won't dye her hair -- when she's 40, what's a realistic range for how it will look? It's a plot point, otherwise I'd just make her mousy.)
ETA: Answered! Thank you all!