"That woman is very pretty, but is too young for him."
Translation:Esa mujer es muy bonita, pero es muy joven para él.
Demasiado only changes according to gender when it's used as an adjective (meaning "too much" or "too many") that modifies a noun. e.g. Tengo demasiadas faldas.
Here, though, demasiado is used as an adverb that modifies the adjective joven. And as with most adverbs, it doesn't change according to number or gender.